Susan and Barbara continue to examine the giant footprint in the snow. Ian joins them outside the TARDIS, surmising that the footprint is an ordinary one simply melted and enlarged by the sun. The Doctor emerges also, but only long enough to say he believes they are on Earth, high up in the mountains. Then he disappears grumpily back inside.

Happy at the possibility of being on Earth, the others try to figure out what mountains they've landed on. Perhaps, Susan suggests, it is the Himalayas, the Roof of the World. They are interrupted when the Doctor bursts out of the ship and announces that a serious fault has developed in one of the ship's systems. There are no lights, heat, or water in the ship!

Ian, ever the optimist, immediately suggests searching the area for some combustible fuel. The Doctor is tetchily sceptical, but Ian persists. Barbara joins him, but Ian and the Doctor force Susan to stay behind. The Doctor sends her into the ship to fetch the damaged circuit while he worries aloud what will happen when darkness falls...and so does the temperature.

Some time later, Ian and Barbara have found nothing. The wind howls and they are cold and tired. Barbara is suddenly startled when she sees what she thinks is an animal stalking her. Ian sees some strange footprints and decides that they should head back for the TARDIS.

Meanwhile, the Doctor has the broken circuit in his hands. His fear has been realized: he must build a whole new one to fix the problem and it will take him several days to do so. Ian and Barbara rejoin him and learn of this. The Doctor suggests moving to a lower altitude, but Ian says he is sure what Barbara saw was a person and not an animal. They should try and make contact. In confirmation of this, Susan spots a caravan of wagons moving in the distance. It is their only chance of shelter. The entire group goes after the wagons.

In the blowing snow, they lose sight of the caravan. However, they are shortly beset by Oriental-looking soldiers who surround them. Their leader, a man called Tegana, refuses Ian's request for shelter. In fact, he believes the travellers to be evil spirits in human guise who want to destroy him and his men. He orders them all killed. However, his order is stayed by a newcomer - a European man. He issues orders in the name of Kublai Khan, seeing that all of the travellers are taken into his camp further down the pass.

In the camp, a young Chinese girl called Ping-Cho waits on them graciously. The Doctor is unwell, suffering from altitude sickness. Food and warmth help to restore his strength. The travellers try to make sense of this hodge-podge of people who have rescued them. While Ian explains the effects of air pressure on boiling points, Barbara uses her knowledge of history to deduce that their host is none other than Marco Polo!

Marco's caravan is headed to Shang-Tu, in Cathay. The Doctor and his friends are welcomed as guests, even though Marco is curious as to why they were out on the mountainside unprotected. Before retiring for the night, the Doctor gets two questions answered: they are indeed in the Himalayan Mountains, and the year is 1289.

Later that night, both Susan and Ping-Cho are restless. They form a bond as they talk. Ping-Cho is curious as to where Susan is from. Susan ducks the question, saying only that she has had many homes in many places. Ping-Cho says that she is from Samarkand and that she travels to Shang-Tu to be married. Susan is astonished as Ping-Cho is just 16, like her. She is even more astonished to learn that the marriage was arranged by Ping-Cho's parents and that she has never met the man. All she knows about him is that he is a very important man and that he is 75 years old!

Meanwhile, Tegana talks with Marco, insisting that he should have been allowed to kill the strangers. Marco believes them to be harmless. Unusual yes, but harmless. Tegana insists that they are evil spirits or human magicians. Their "carriage" has no wheels and it is far too small for 4 people to fit inside, yet they all did.

The next morning, Marco and Tegana bring Ian and Barbara up the mountain to where the "magic caravan" sits. Marco is sceptical as to the value of the object until Ian tells him it travels by "flying". Marco has seen Buddhist monks make objects fly and he believes that it can be done. He is impressed, but Tegana takes it as proof that the strangers really are evil spirits.

Marco shows great interest in the TARDIS and tries to get in. Barbara and Ian tell him that only the Doctor has the key and that only the Doctor can make it fly. They also tell him it is damaged and in need of repair. Marco makes plans to have the caravan brought down the pass and into the camp.

Meanwhile, the Doctor has recovered from his altitude sickness overnight. He joins Ping-Cho as she cooks some bean sprout soup. He engages her in conversation and learns that Tegana is a Mongol Warlord, an emissary from the Lord Noghai, who has been at war with Kublai Khan. The war is now over and Noghai has sued for peace. Tegana is travelling to the Khan's court to discuss armistice plans. The Doctor is sceptical that such a violent man as Tegana would made a good peace emissary.

Marco and the others return and the Doctor learns that the TARDIS will be brought to the camp. At first, he is pleased, certain he can now repair the circuit in only a day or two. However, Marco says that his caravan must move on immediately. They cannot stay up here, but they will stop for a few days in the town of Lop. They and the TARDIS will go with Marco's caravan. The Doctor hopes to work on the circuit as they proceed, but Marco again says no. In order to appease the superstitious Mongols, no one must enter the TARDIS. A little warily, the Doctor agrees to this. No one will enter the TARDIS until they reach Lop.

However, the Doctor is right to worry. Marco keeps a journal of his travels and he reveals his deception as he reflects upon the events in his diary:

“My plan has worked. The strangers and their unusual caravan accompany me to Lop. Our route takes us across the Kashgar Valley and south-east to Yarkand. Here we join the old silk road along which the commerce and culture of a thousand years has travelled to and from Cathay. I wonder what the strangers' reaction will be when I tell them what I propose to do?”

Arriving in Lop, the caravan stops at a way-station where everyone is quartered. Marco explains that these stations are placed all throughout Cathay by the Khan to aid travellers. For those in his service, like Marco Polo, they provide shelter, provisions, horses, anything his servants may need. Shortly, the TARDIS arrives on its wagon and is set up in the courtyard. Happily, the Doctor goes to enter it, anxious to begin his repairs. However, he is stopped by two of Tegana's warriors. Angrily, the Doctor turns to Marco for an explanation.

Marco tells them that he arrived at the Khan's court 18 long years ago with his father and uncle. He has been in the Khan's service ever since, travelling to every corner of his vast domain and beyond. Two years ago, he and his family asked permission to go back home to Venice. But the Khan refused to part with such able servants and would not let them go. Marco explains that he truly desires to return home and that he plans to ask the Khan again when they reach Shang-Tu, but he plans to accompany the request with a gift so magnificent that the Khan cannot refuse: the Doctor's "magic caravan".

The Doctor, of course, thinks him mad but Marco persists. He promises to take them all back to Venice with him where the Doctor can "make another" caravan. The travellers all try to explain that it is impossible to build another TARDIS, but Marco makes it clear that he is desperate. He wants to go home and it must be now. If the old Khan dies before granting permission, Marco will never be able to leave in the resulting chaos.

Ian tries one last time to convince Marco that only the Doctor can fix or fly the "magic caravan", but Marco's desperation is so absolute that he refuses to listen. This gift will make the Khan the most powerful ruler in the world, and Marco will have his freedom. Marco barges out, leaving the time travellers to ponder their fate. The Doctor reacts oddly, collapsing in a fit of almost hysterical laughter. He understands the situation perfectly, but cannot control his laughter.

"I have taken charge of the travellers' unusual caravan, and set out into the Gobi desert. The journey across this vast ocean of sand is slow and hazardous. To make matters worse, the old Doctor continually shows his disapproval of my action by being both difficult and bad-tempered. For three days now, during which time we have covered no more than 30 miles, I have had to endure his insults.”

The third night of their journey, Ian and Barbara converse with Marco and Tegana about their water situation. Marco is certain they have enough water to get across, as long as nothing goes wrong... Marco produces an intricately-decorated chess set and proposes a match with Ian. As they begin to set up the board, Susan, who has been sitting quietly, leaves the tent suddenly. Concerned at this, Barbara excuses herself and goes outside as well. Tegana watches the preparation of the game, fascinated by its similarity to war strategy.

Barbara finds Susan sitting just outside the tent, staring into the star-filled sky. She, like her grandfather, is angry with Marco for taking the TARDIS away and stranding them here in this primitive time. What is worse for her is that the Doctor has kept to himself the entire journey so far, sulking and sniping. She feels very alone. Barbara tries to comfort Susan, reminding her of the Doctor's resourcefulness and telling her that they will soon be off again. Susan wants very much to be gone, but Barbara cleverly reminds her that the sooner they are on their way, the sooner she and Ian will reach home and have to leave the TARDIS. Susan realizes that she doesn't want this to happen too soon and that this situation delays their inevitable separation. Susan is cheered up and heads off to bed, stopping for a moment to see Ian's progress against Marco. It is not going well for Ian.

However, Susan is still restless when she reaches her tent. Ping-Cho is also awake, contemplating the beauty of the desert. She tells Susan that the moon will rise in a few hours, turning the sand into a great sea of silver. Both girls want to see this and plan to rise in a few hours to do so.

In the main tent, the chess match continues. Ian manages to put Marco's king in check, but Marco responds in kind. The game continues, watched closely by Tegana.

Two hours later, Susan and Ping-Cho emerge carefully from their tent to view the moonlit desert. Both are captivated. As Susan tries to explain to Ping-Cho the meaning of the term "crazy", they spot Tegana heading out of the camp alone. He stops to chat with the guard and then is gone into the desert. Impetuously, Susan decides to follow him. Ping-Cho tries to stop her but is dragged along with her new friend.

In the camp, silence reigns. Ian and Marco sleep in the main tent. However, the stillness is soon broken by the sounds of the horses growing restless. They paw the ground and whinny nervously. The noise wakes first Marco and then Ian. Marco knows instantly what it means: a sandstorm is coming. He heads off to secure the horses.

Susan and Ping-Cho must hurry to keep up with Tegana. He is far ahead and moving quickly, striding with a purpose. The heat seems very intense and breathing is more difficult. They stop to rest, collapsing on the ground and quickly realizing they should never have gone out. But just as they decide to turn back toward camp, Ping-Cho calls for silence. In the distance, and gaining fast, is the sound of swirling wind. On the horizon is a dusty cloud. Ping-Cho realizes it is a sandstorm and starts running for camp. Susan follows, but knows they are too far away to make it in time. The swirling wind catches them out in the open. They scream as the sand hits them. The strange sound of the rushing wind engulfs them and their screams.

The sounds of the storm can also be heard in the camp now. Barbara has awakened and joined Ian and Marco in the main tent. Both Ian and Barbara are chilled by the sounds of the howling wind, but Marco explains that it can also sound like singing, or music, or a familiar voice calling your name. He is fascinated by it, although respectful of its dangers.

Notably absent are the Doctor and Tegana. The Doctor is still asleep, Ian reports, but no one has seen Tegana. Marco thinks it unlikely that he is out in the storm, but knows there is nothing they can do even if he is. Also absent are Susan and Ping-Cho. Barbara goes to check on them and finds their tent empty. She returns to Ian and Marco in hysterics. Marco insists they cannot go out to look for the girls during the storm, but Barbara continues to plead while Ian tries to calm her down.

Out in the desert, the "singing" of the sand continues. Susan and Ping-Cho have found a bit of shelter behind a sand dune and are waiting out the storm. However, Susan thinks she hears Ian's voice in the wind, calling her name. She breaks free from Ping-Cho and stands up to call out to Ian. Instead, she screams as she sees Tegana standing over her.

Ian is still in the camp, begging Marco for permission to go in search of the girls. He will not let anyone go until dawn when it is light enough to see.

After what seems like an eternity, the storm subsides. Barbara is angry and worried, impatient to get out and start searching. Marco tries to calm her, assuring her that they will not break camp here until the girls are found. However, their worry is dissipated when Tegana enters the main tent with Susan and Ping-Cho. All are astonished. Barbara scolds Susan and asks where she's been. When Susan tells her they simply went for a walk, Marco becomes very angry. He also turns on Tegana, assuming that he accompanied them, but Tegana says he was out walking independently and only found the girls by chance hiding from the storm. He says innocently that the guard knew he had gone. Marco is angry again, ordering everyone to notify him if they wish to leave the camp for any reason. He orders Tegana and the girls to prepare to break camp. Barbara tries to convince Marco to remain here for another day, to recover from the shock of the storm, but he insists they must move on. They cannot afford to waste their water without making progress across the desert.

Tegana listens to this from outside the tent. His plan to poison the water unsuccessful, he must decide what to do next. Marco is none the wiser.

“Progress today has been good, although we are all very tired after a sleepless night. How can I ever repay Tegana for saving Ping-Cho and Susan? We covered 15 miles before I gave the order to set up camp for the night.“

Once encamped, Susan and Ping-Cho have the time to discuss the harrowing events of the previous night. Susan is certain that Tegana is not the type of person who goes for a walk simply because it's a nice night. Ping-Cho agrees it seems unlikely, but neither of them can figure out why he would lie about why he was walking in the desert.

Meanwhile, Tegana is polishing his sword while Marco writes in his journal. Tegana seems mildly interested in this activity, but he waits until Marco is fully engrossed and then yells out Marco's name just before lunging at him with the sword. Marco barely manages to repel the blow in time. Pleased, Tegana heads off to see to the horses and give the night guard his instructions. Marco insists the guard be sent to him for orders instead. It is a strange power struggle between these two. However, instead of going to the guard, Tegana first goes to the wagon where the water gourds are stored. He slashes open every one in the wagon and watches as the water pours out onto the ground.

The next morning, the vandalism is discovered. Marco is sure that it is the work of bandits. It is a common occurrence in the desert - the bandits then wait until the caravan is weak through lack of water and attack. Marco says they have only enough water for 3 or 4 days, even if they ration it. It may be enough to get them to Lop, but Marco is sure the bandits will be waiting for them in that direction. The only other possibility is an oasis north of the camp, but it is 5 or 6 days' journey away. They may not make it. But it appears to be their only choice and Marco decides to head for it. Tegana at first refuses to join in, knowing they will not make it before the water runs out, but this is just a ploy to leave the caravan and rejoin his Mongol warriors. However, Marco lays down the law as commander of the caravan and in the name of Kublai Khan they all go north together.

But the trip is more difficult than Marco imagined.

“Have I made the right decision? Each day our progress towards the oasis becomes less. On the first day we covered 20 miles. On the second 15. The third, ten. The fourth day's total was eight. Now on the fifth day we have travelled only two miles before the heat of the sun has forced us to stop. We are nearly exhausted and our situation is perilous.”

The travellers have taken refuge in the shade of their wagons as the sun beats down on the desert. Marco rations out the very last of the water to everyone. It is pitifully little. Everyone is exhausted. Marco cannot or will not say how much farther they have to go. Obviously, it is quite a distance. Tegana offers to go ahead alone to the oasis to bring back water. He still has some strength left and his horse is still the strongest. With no other choice, Marco agrees and Tegana swiftly mounts his horse and rides off.

Marco plans to continue moving toward the oasis, so that Tegana's return trip will be shorter. However, just as they prepare to break camp, the Doctor collapses from exhaustion. Marco insists they go on and suggests preparing a place for him in one of the wagons, but Barbara suggests that the TARDIS would be more comfortable and less bumpy for him as they go. Reluctantly, Marco agrees. Susan and the Doctor may go into the TARDIS, but the others must remain with Marco. Ian expresses his gratitude, but is more concerned that the Doctor will not last another 24 hours without water. Marco agrees, saying that their fate rests with Tegana.

Daylight is growing brighter as the exhausted travellers lie in the shade of a wagon. There is no sign of Tegana, their only hope. They know they must rouse themselves to try and go to meet him, not knowing that he has no intention of returning to them. However, they are far too tired to move. They may never move again.

Their work completed, Susan and the Doctor emerge from the TARDIS, each carrying a container of water. The existence of the precious water rouses the entire group, as they realize they are saved. However, Marco quickly turns angry, believing that the Doctor has concealed the water from him all this time. The Doctor tries to explain but Marco cannot understand the term "condensation". The Doctor turns to Ian to explain the concept, which Marco eventually grasps. Still suspicious though, he orders Ian to drink first. He does so, and survives.

“My conscience pricks me. I was adamant, despite the Doctor's protests, that the key and his caravan should be handed over to me. Now we journey across this burning desert and I shall not rest until I see the temple spires of the city of Tun-Huang.”

Elsewhere in the way-station, the Doctor tells Ian that he will be able to start work on the TARDIS' damaged circuit as they will be staying here for a few days. Hopefully, Marco and Tegana will be less vigilant while stopped here. The Doctor produces a second TARDIS key, to Ian's amazement. The Doctor made it in secret during their night in the oasis. Ian is impressed at the Doctor's guile. Susan joins them to let them know that Ping-Cho will soon begin her story of the Hashashins. In much better spirits now, the Doctor cannot wait to hear it.

Ping-Cho - now elegantly dressed in robes for her presentation - enters to a polite round of applause. She bows courteously and then begins her recital:

Appreciative applause fills the room as Ping-Cho ends her tale. The Doctor, Marco, and Ian congratulate her. Ian tells Susan that the word Hashashin is still in use in English in the 20th century: "assassin". Susan wants to know if the lair of Ala-eddinn is really nearby, but she is told it is in Persia. However, some of Ala-eddin's followers did live in the Cave of 500 Eyes, very nearby.

As this discussion goes on, Tegana slips out of the room, noticed only by Barbara. She has been watching him intently the whole time. She, too, slips out and follows him.

Tegana goes directly to a cave outside Tun-Huang - the Cave of 500 Eyes. There, in the darkness, he meets a comrade called Malik. Tegana has come to receive news from his Lord Noghai. The two men retreat into a secret chamber at the back of the cave to conduct their business. As they go, Barbara enters the cave. She must feel her way in the darkness, uncertain what she is heading toward.

In the secret chamber, lighted by torches and lamps, Tegana is greeted by another Mongol comrade - Acomat. He bears the news that Noghai has formed his army and is marching toward Karakorum. There he will wait for news of Tegana's "success". Acomat asks why Tegana did not meet their courier on the third night in the desert and Tegana blames the sandstorm.

Not liking the direction of the conversation, Tegana is only too glad to talk of the gift he plans to present to Noghai: the magic caravan that flies. He instructs Acomat to attack Polo's caravan as it travels toward Shang-Tu, capture the magic caravan and kill Polo and all the others. Then Noghai will be invincible. Malik enters from his guard duty, reporting that there is a woman in the outer cave. Suspicious, Tegana draws his sword and goes out.

Tegana, familiar with the cave, has the advantage over Barbara in the blackness. He manages to get past her in the dark as she feels her way along one wall. He grabs her from behind and she screams.

Later, back at the way-station, Tegana plays along when Barbara's disappearance is discovered. Marco is furious at her leaving the station, but agrees to a search of the city. He despatches Tegana to one section, while he and Ian take the other. The Doctor and the others are ordered to stay in the way-station.

Unfortunately, Barbara is far from the city, tied up in the cave's inner chamber. Malik and Acomat stand guard over her, playing dice while they wait for Tegana to return.

Back at the way-station, the Doctor has taken advantage of the chaos and gone into the TARDIS to work. As he comes out, Susan and Ping-Cho rush up to him. The Doctor is worried that Ping-Cho will reveal his second key to Marco, but Susan assures him that he can trust her. She tells him they may know where Barbara is. She had been very interested in the Cave of 500 Eyes when Marco talked about it earlier. The Doctor thinks this plausible and turns to Chenchu, the landlord, to find out where the cave is. Chenchu explains it is only a mile away, set back against the foothills, but he warns against going there at night as the cave is haunted by the spirits of the Hashashins. The Doctor laughs this off, determined to go. Susan and Ping-Cho encourage him, and they plan to accompany him. Despite Chenchu's warnings, they head off.

They find the cave easily and enter, the faint glow from the Doctor's oil lamp sending giant shadows skittering along the walls. They are drawn to the faces on the wall and the Doctor quickly determines the origin of the cave's name: the eyes on the faces are carved out of quartz, which reflects light and makes the eyes stand out grotesquely. Susan draws their attention back to the task at hand and they start their search for Barbara.

At the way-station, Tegana returns after his "search" of the city. Chenchu immediately reports to him that the Doctor and the girls have gone - after he came out of his magic caravan. Tegana is strangely pleased at this news, but grows angry when he learns that the Doctor and the girls went to the Cave of 500 Eyes.

The Doctor's search has been fruitless. He thinks this a fool's errand that has taken precious time he could have used repairing the TARDIS. He is certain Barbara is safe in her bed at this very moment, but Ping-Cho finds a handkerchief on the ground which Susan identifies as Barbara's. With renewed hope, the girls begin yelling loudly for Barbara.

In the secret chamber, Barbara and the guards hear the noise. Barbara struggles, but is bound and gagged. Acomat gets up to investigate.

Marco and Ian return to the Tun-Huang way-station after searching part of the city for Barbara. Tegana is not there, neither is the Doctor, Susan, or Ping-Cho. They learn from Chenchu that the Doctor and the girls went to the Cave of 500 Eyes, and that Tegana followed them. Marco is of course furious that they have left the way-station, but Ian tells him there will be time for anger later. Now, they must get to the cave.

In the cave, the Doctor is examining the wall where Susan saw the moving eyes. He thinks Susan's claim is rubbish - all the eyes are quartz. They are interrupted by the arrival of Tegana, who laughs at the idea that Barbara might be here. But when the Doctor shows him the handkerchief he found, Tegana's bravado turns to nervousness. He glances briefly at what he calls "the passageway" where the handkerchief was found and then invokes stories of the evil spirits here in the cave. They should leave immediately. The Doctor assures him he is not afraid of ghosts and wants to get on with the search, especially when Ian and Marco join them a moment later.

Ian takes in the situation quickly - he, too, is sure that Barbara was here. Tegana repeats his fearful claim that the cave is haunted, trying to scare the others away. Ping-Cho joins in innocently, telling Ian how Susan saw the eyes move. Intrigued, Ian investigates and becomes excited by what he finds. Knowing what it is, Tegana utters a loud cry to the spirits of the dead Hashashins to "spare them". Unimpressed, Ian shows Marco and the others the face Susan indicated. The quartz eyes are not carved from the wall, but set in from behind and are removable. There is some secret room behind the wall, complete with peepholes.

It takes Ian only a second to find the door and its opening mechanism. The door swings open to reveal the old Mongol Malik just about to kill Barbara. He freezes when the door opens, looking at Tegana. Tegana leaps forward and in a split second kills the old man. Ian unties the nearly-hysterical Barbara, comforting her. She is safe now.

Some time later, back at the way-station, Marco praises Tegana for saving Barbara. Tegana wonders why he cares - why he doesn't just leave these strangers behind as they've been such trouble to him from the start. But Marco is honour-bound to see them safely home after taking their caravan away from them. Marco and Tegana agree that the Doctor does not hold Marco in such esteem, but Tegana also insinuates that Susan has managed to draw Ping-Cho from his side. Polo insists that the travellers are harmless, despite Tegana's vehement disagreement. The Mongol warlord predicts with certainty that they will use deceit and the power of the magic to destroy Marco and everyone else. He then tells Marco that the Doctor was in his caravan this very night, which shakes Marco's faith.

Barbara, Ian, Susan, and Ping-Cho join them and Marco is quick to address Barbara as to why she went to the cave when she knew it to be dangerous. She tells him that she followed Tegana there, which the warlord hotly denies. Although it sounds ridiculous, Barbara sticks to her story. Tegana strolls off, reminding Marco of his dire prediction.

Tegana's insinuations have worked, lodging doubt about the travellers in Marco's mind. He orders Ping-Cho move her belongings to another room. She will no longer be allowed to room with Susan. Both girls are distraught by this news, but Marco stands his ground. Tegana, out of sight nearby, hears this and smiles.

“Poor Susan and Ping-Cho. And yet what alternative had I but to separate them? Now my caravan seethes with suspicion and discontent as we journey southwest. The route takes us to the ancient cities of Su-Chow and Kan-Chow, where the Great Wall of Cathay begins. Following the Wall we travel south to Lan-Chow which lies by the banks of the Yellow River. Here our route swings north and with the river always in sight we journey toward Shang-Tu.”

The caravan stops for a time at the way-station of Lan-Chow. The Doctor laments his lack of progress on the TARDIS circuit due to Marco's change of attitude. Still, with a bit more time - and a bit of luck - he should be done in another night. Ian and Barbara are overjoyed with this news, anxious to be gone from this time and place. Barbara is, however, sad for Susan. She and Ping-Cho had formed such a fast friendship that their separation has been hard on them. The sudden departure will be even worse. For his part, the Doctor has been suspicious of Ping-Cho ever since she found out about the spare TARDIS key. He will have no qualms about leaving - the sooner the better.

Elsewhere, Susan sneaks into Ping-Cho's room, making sure no one is following her. Ping-Cho is very sad, trying to figure out what Tegana could have told Marco to make him split the girls up. Susan wishes they could prove that Tegana was lying about never having been to the cave before Barbara was found, but Ping-Cho doesn't really care. She is sadly certain that the Doctor must be nearly finished with his work and they will all be leaving soon. Susan tells her that he has finished, but she promises to come and say good-bye before they leave, no matter how late it is. Cheered a little, Ping-Cho promises once again to say nothing to anyone about the Doctor's spare key. However, Tegana has happened by in the hallway outside and stopped to listen to the voices. He now knows about the second key.

“For the past three days I have followed the course of the Yellow River as it flows north to the small town of Sinju which lies nestled against the Great Wall.”

The caravan stops at Sinju for a short rest. Tegana asks Marco if he can go into town and Marco agrees. Thinking nothing of it, Marco continues about his business.

Elsewhere in the way-station, a troubled Ping-Cho suddenly remembers something about the night in the cave. She rushes to find Susan to tell her that she can prove Tegana had been there before Barbara was found. Marco enters the way-station to find the two girls talking excitedly together. They tell him they can prove that Tegana lied. He tells them that this is a serious charge and asks for their proof. Ping-Cho reminds him of Tegana's use of the word "passageway" to describe the dark area where Barbara's handkerchief was found. Until Ian found the secret door, however, no one could have known it was a "passageway", unless he had been there before. Marco reacts angrily. To him, this is no proof at all. He dismisses the story and tells them it will take much more to shake his confidence in Tegana.

Meanwhile, the duplicitous Tegana is meeting with Acomat in the town of Sinju. Acomat is angry at yet another missed opportunity to kill Polo and capture the magic caravan, but Tegana is all business. Soon Marco's caravan will set out across the bamboo forest. On the second night, Tegana will deal with the guard and signal to Acomat with a burning torch. Then the Mongol warriors will attack the camp and kill everyone. Acomat is a little sceptical of being able to kill "the old magician", but Tegana is certain a stake through the heart will do the trick.

In the courtyard of the way-station, the Doctor decides to take a terrible risk - completing the work on the TARDIS while it is still light. Leaving Barbara on guard outside, he dashes inside. But Tegana emerges from the way-station at that moment and sees the TARDIS door close. He rushes back inside the way-station. Panicked, Barbara hurries off to find Ian.

Inside the ship, the Doctor sets about his work.

Ian and Barbara return to the courtyard well ahead of Tegana. At least they have a chance. Ian is sure Tegana has gone to fetch Marco. He heads off to find them both and stall them while Barbara is to get the Doctor out of the ship. But the Doctor is absorbed in his work placing the repaired circuit and does not hear her knocking and calling.

Ian is lucky enough to find Marco before Tegana has. He begins asking about why Marco ordered Susan and Ping-Cho separated. Marco says Susan is a bad influence: Ping-Cho now supports the strangers against Tegana. Ian says this is because she knows they are telling the truth, but Marco explains that Tegana is a powerful and highly-honoured emissary of Lord Noghai while Ian and his friends are mysterious travellers from some unknown land they will not speak of. He has no choice but to stick by Tegana. Ian tries to argue this point, but Tegana finds them just then and tells Marco that the Doctor in inside his caravan. Marco is furious and turns to Ian for confirmation. With no other choice, Ian suggests they go and see.

When they reach the courtyard, it is deserted except for the TARDIS. Tegana insists the Doctor got inside with a second key, and Ian says they must then wait to see if he comes out. He is trying to bluff, hoping they won't wait, but Tegana stakes his reputation that he is telling the truth. Unfortunately, their wait is short as the Doctor exits the ship a moment later. He only barely manages to close and lock the door when he realizes what is happening.

Marco's faith in Tegana is back in full force and he demands the Doctor's second key. For a moment, the Doctor resists but Marco prizes the key from his hand. Angrily, the Doctor warns Marco not to try the key. Without proper knowledge of the ship and its systems, Marco would destroy it if he tried to open it and the Doctor is willing to let that happen rather than let Marco inside. Furious, Marco has guards drag the Doctor and Ian away. The Doctor hurls insults at Marco as he goes. While Marco seethes with anger, Tegana cements his suspicions by asking which of them is the true troublemaker.

“What a nightmare this journey has become. Our progress is impeded because Tegana, the bearers and I must constantly be on the alert for any signs of trouble from the prisoners, and Ping-Cho's resentment of me only serves to make my task more difficult. But I have succeeded in keeping her away from them by setting up a separate tent for the Doctor and his companions.”

On their second night of rest in the bamboo forest, the travellers are being held under guard in their tent. Ian's frustration is clear as he throws and breaks a plate on the ground. Everyone is on edge, especially since the TARDIS circuit has been completely repaired and installed. All they must do now is get inside.

The Doctor is sure that Ping-Cho told Marco and/or Tegana about the second key, but Susan refuses to believe it. Ian cuts short the argument by stressing the need to think about getting the keys back. The Doctor suggests capturing Marco and this idea appeals to Ian. At night, he has only one guard and the travellers are just desperate enough to succeed. To escape from the tent, Ian uses the jagged edge of the broken plate to cut through the back wall. He will deal with their guard and return for the others. Confidence returns to the group.

Hidden in the bamboo forest, Acomat and his Mongol warriors await Tegana's signal to attack. They have Marco Polo's camp surrounded.

Ian beckons the others out of the tent and shows them the dead guard. Barbara concludes there must be bandits ready to attack. The Doctor suggests continuing with their escape plan, but when Susan spots Tegana leaving his tent, Ian knows they are already too late. He goes to do the only thing he can - warn Marco.

Ian enters Marco's tent, trying not to startle him. However, Marco immediately reaches for his sword as soon as he wakes up. Ian stops him and explains quickly what is happening. Marco asks where Tegana is and Ian tells him he is outside, but unaware of the danger as yet.

They find him next to the fire, before he has had time to light his torch. He is standing there as if guarding the fire. Marco instructs him to wake the bearers and prepare to defend against a bandit attack. Tegana wonders aloud why Ian is free, but Marco dismisses this for the moment. Marco sends Susan and Barbara to wait with Ping-Cho in her tent, where they will be safer.

Tegana returns, having armed the bearers and positioned them on the perimeter. Marco arms the Doctor and Ian with swords, not sure they will even be able to use them. The Doctor thinks this method of defence foolhardy and suggests they use the TARDIS to get themselves to somewhere safe. Tegana scoffs at this trick, sure that "the bandits" won't attack. He even suggests that the strangers killed the guard themselves to create this panic. Marco is unsure, but he has the keys and will not hand them over. The Doctor mocks Marco, certain they will all be killed by the bandits if they don't use more "enterprise". Ian agrees, coming up with a plan to heat some bamboo on the fire, which will expand and explode. The noise should be enough to frighten the bandits off.

In the darkness of the forest, Acomat and his warriors grow worried over the lack of a signal. Soon the moon will rise and they will not have the advantage of darkness. They worry that Tegana has failed them again. Acomat decides that they will attack the camp at the moon's first light even if there has been no signal. And without the advantage of darkness, they will abandon stealth for an all-out attack.

As Ian and Marco collect bamboo for the fire, Marco wonders again about the nature of these strangers: are they friends or foes, magicians or humans? Marco asks Ian if they had been trying to escape and Ian admits the whole plan. Marco seems to believe that they would not have harmed him, but vows to be more careful in future. Obviously the TARDIS is very important to both the strangers and to Marco, but which will prevail is still to be decided.

Ian and Marco note the imminent rising of the moon, certain that the attack will come soon despite Tegana's insistence to the contrary. They take a gamble and begin piling bamboo on the fire. Ian wakes the Doctor and they prepare. They are just in time as "the bandits" rise up and move in on the camp.

With a mighty yell, the Mongols descend on the camp with swords drawn. The bearers manage to hold back the majority of the warriors, but several - including Acomat - get through to the centre. Acomat sees Tegana and greets him scornfully. Horrified that Marco and the others might hear, Tegana rushes forward and kills his kinsman with one blow. Then the bamboo begins to explode one after another. The Mongol warriors, hearing "devils" in the fire, break off their attack and run into the forest in terror.

Barbara, Susan and Ping-Cho emerge from their tent and join the others to celebrate the victory. Marco is amazed at the ingenuity of the strangers and at how their ruse caused the "bandits" to flee. However, the Doctor notes Acomat's body lying near Tegana and says that it was the death of their leader that caused them to break up and run so readily. Tegana takes the insinuation personally, which is exactly what the Doctor wishes him to do.

“At sunrise we buried the dead, broke camp, travelled to the edge of the bamboo forest and out beyond it in the plain. But, at midday as we were all tired after the events of the previous night, I ordered a halt.”

As they rest, the Doctor, Ian and Barbara cannot help but fume over Marco's treatment of them. They saved his life, yet he still will not let them go. All agree that, difficult as it may be, they must go on trying to get the keys and escape. It appears that Marco has thought the same thing, as he approaches the group and asks for their word of honour that they will not attempt another escape. Their silence is a clear response. The Doctor explains bitterly that they sacrificed their freedom last night to save him. But this has not been lost on Marco. He revokes their house arrest and agrees to let Susan and Ping-Cho keep company again. However, the TARDIS keys will remain with him and he will be under constant guard. Tegana, not pleased with this turn of events, offers to be Marco's personal guard. Marco categorically refuses and strides off.

Tegana hurls a veiled threat against the strangers and then follows Marco. Ian is sure he knows they are on to him. They try to reason out Tegana's actions, hoping to uncover a motive. The Doctor is sure that Tegana orchestrated the attack on the camp and killed the leader to silence him. This information jogs Barbara's memory and she remembers Acomat from the Cave of 500 Eyes. The Doctor is sure Tegana wants the TARDIS, but Ian and Barbara think he is after more - maybe the life of Marco Polo as well. The Doctor urges caution in their line of thinking.

Their conversation is ended when Susan and Ping-Cho join them, escorting a young man. He is Ling-Tau, a courier from Kublai Khan himself. Ping-Cho goes off to fetch Marco while Ling-Tau rests. It is clear he has travelled far - all the way from Shang-Tu, he tells them.

Ping-Cho enters Marco's tent just as he finishes writing in his journal. She cannot help but see him hide the TARDIS keys in the spine of the book. She delivers her message and turns to go, but Marco stops her, making her promise to tell no one where the keys are hidden. Reluctantly, she agrees.

The Doctor and his friends converse with Ling-Tau. They learn that he has ridden 300 miles in little over 24 hours with no sleep, changing horses every 3 miles. They are all impressed by this impossible feat of physical endurance, but the courier simply says that horsemanship runs in the blood of his people.

Marco joins the group and receives the message from Ling-Tau. He reads it quickly and announces that they must leave at once as the Khan wishes to see him as soon as possible. They have 6 more days' travel to the city of Cheng-Ting, then they can continue on horseback to Shang-Tu. However, their possessions - including the TARDIS - must travel on a separate trade caravan from Cheng-Ting. The travellers will be separated from their ship, perhaps permanently.

“My one concern now is to reach the summer palace as quickly as possible. So, within the hour, we were on the move again and on the sixth day of our journey the spires of Cheng-Ting could be seen in our horizon. By later afternoon we had arrived at the way-station of the White City as Cheng-Ting is often called.”

At the way-station, Marco must deal with the fatuous landlord, Wang-Lo, as well as an agitated Doctor. The TARDIS has not been set up in the courtyard as promised and he wants to know where it is. Wang-Lo explains that he had it moved to the stables for safety reasons. Marco inquires as to the next commerce caravan for Shang-Tu and is told that is leaves in two days' time. All of their baggage - including the Doctor's "caravan" - will go along with it. The Doctor is not pleased but must accept it for now.

Elsewhere, Susan and Ping-Cho sit idly next to a fish pond. There is an undercurrent of great sadness as the girls watch the fish and match their personalities to all of their friends. Suddenly, Ping-Cho's tears break through and she tells Susan how much she misses her home. It seems so far away and she has finally realized what Susan has been saying: her mysterious home is even farther away, unreachable even from far-off Venice. If they do not have their caravan, Susan will also never see her home again. Ping-Cho admits that she knows where the keys are hidden but that she promised not to tell. Susan assures her that she will never force her friend to break her promise.

At the stables, Tegana meets with an evil-looking man called Kuiju, a petty criminal. He wears an eyepatch and carries a monkey on his shoulder. Tegana is arranging to have the TARDIS stolen from the station and brought to him later. Kuiju wants to be paid in gold and Tegana promises 50 pieces tonight and 50 more when he receives the caravan. They are to meet in the Street of the Beggars at midnight to seal their deal. Kuiju goes off, ready for an easy job. Tegana looks over the TARDIS, soon to belong to his Lord Noghai.

Later, in Marco's room, Ping-Cho enters to inform him that it is time for dinner. He puts away his journal and notices ink on his hands. He goes off to clean them, leaving Ping-Cho to go on ahead. The girl does not hesitate a moment before she goes to Marco's journal and removes one of the keys there.

After dinner, Ping-Cho meets up with Susan in the courtyard. There she gives Susan the key she stole. Susan is astonished, but Ping-Cho is happy. She did not break her promise to Marco - she told no one where the key was hidden. Even though he'll be terribly angry, Ping-Cho will be happy to know that at least Susan will be able to get home, even if she herself will not.

The two girls embrace, the sadness of their impending separation starting to weigh on them. They must hide their tears and the key as Tegana passes by, bidding them good night. Susan starts to head off to tell the others, but Ping-Cho stops her and makes her promise to come and say good-bye before she goes. Susan promises, her eyes filled with tears of gratitude and lost friendship.

The travellers must wait until nearly midnight before everyone else is asleep in the bustling way-station. They slip out of their rooms and into the stables. They stop when they find a guard on duty near the ship. Ian spies a goblet on the ground and hits upon a plan. He fills the goblet with water and pretends to be drunk, staggering good-naturedly up to the guard. When he offers the guard a drink, his face brightens and the two men go off together.

The Doctor and Barbara take the opportunity to go to the ship and unlock the door. Ian suddenly turns on the guard and knocks him unconscious, then joins the other two. All three enter the TARDIS, unaware that Susan has wandered off to find Ping-Cho.

Inside the ship, The Doctor prepares to leave, but Ian realizes Susan is gone. Barbara knows instantly that she's gone to see Ping-Cho and the Doctor erupts in a fury. Ian still thinks they have a chance as long as the guard is still unconscious. They wait a moment longer for her.

Susan is not far away from the TARDIS and the Doctor, Ian, and Barbara watch her capture on the scanner, unable to prevent it. Ian leaves the ship and approaches Tegana. Tegana refuses to release Susan or to let the travellers go on their way. He orders the Doctor and Barbara to come out, tightening his grip on Susan.

Somehow Marco has been alerted to the altercation and arrives just as the Doctor and Barbara exit the ship. Marco takes in the scene and believes Tegana when he says he captured Susan to keep the others from escaping. The Doctor orders Marco to have Susan released, but he will not do so until the Doctor hands over the stolen key. Tegana is furious over Marco's lack of punishment. He wants to kill them all and be done with them, but Marco refuses. He does not think they will have time to escape again before they arrive at Shang-Tu.

Susan apologizes for thwarting their escape, but the Doctor is only glad that she's all right. Marco questions them as to how they got the key and Ian quickly confesses to having stolen it from Marco's room the night before. Marco seems to accept this lie, but his gaze does not wander far from Ping-Cho.

“A day of hard riding. We left Cheng-Ting at dawn and by dusk had covered forty miles. As this is a densely populated area of Cathay accommodation is not hard to find and we have stopped for the night at an inn. Our baggage, including the Doctor's caravan, is following on with a trade caravan.”

At the inn, Ian and Barbara find a moment to joke about their horseriding ordeal. Apparently the Doctor has been bumped and bruised pretty badly along the way. Barbara turns again to the subject of getting the TARDIS back and forces Ian to talk to Marco yet again. The argument, however, is the same. Both Marco and the travellers need the "caravan" to get home, and Marco cannot believe that their home is unreachable from Venice. The Crusaders came to this land from England. Surely these strangers can return there just as easily.

Trying one last time, Ian tries to explain that they are from the future - that the TARDIS can traverse time as well as space. Marco is incredulous, despite all the wondrous and seemingly impossible things he has seen. A caravan that flies, yes. One that transcends time, no. Ian swears he is telling the truth, but Marco puts his veracity to the test. He asks where specifically Ian found the TARDIS key, and when he cannot answer, Ian's lie is confirmed. Marco doesn't care why Ian has lied, just the fact that he did puts all his words into doubt. It is a pity too, as Marco says he would hand the key over if he truly believed Ian came from another time.

On the stairway nearby, Ping-Cho has left her room and has overheard the last part of this conversation. She goes back up to the room she and Susan share, where her friend still sleeps. With sadness and fear in her eyes, Ping-Cho whispers goodbye and departs.

The next morning, the group has discovered her disappearance. A search of the inn and its surroundings turns up nothing. Ian is certain she's run off because Marco discovered the truth about her and the stolen key. Susan is forced by Marco to admit she thinks Ping-Cho is trying to return to her home in Samarkand. Marco is furious, knowing the journey is impossible for one young girl alone on a road filled with bandits and cut-throats, but he is conflicted about what to do. He is responsible to the Khan for her safety, but he has also been ordered by the Khan to ride to Shang-Tu without delay.

Ian solves the problem by offering to ride back along the road and try to find Ping-Cho. She can't be that far away and someone must find her before she gets into trouble. Marco immediately asks what Ian hopes to gain by this gesture, but Ian refuses to answer the insulting question. Marco agrees and Ian sets off to prepare a horse. Barbara is confident he will succeed, but Tegana warns Marco that this is some sort of ruse.

Back at the Cheng-Ting way-station, Wang-Lo breaks off his dealings with an important Chinese lady to attend to a newcomer. Unbeknownst to him, it is the bandit Kuiju, posing as the commander of the trade caravan to Shang-Tu. He produces forged orders to pick up the TARDIS and Wang-Lo directs him to the stables before returning to other customers.

Before Kuiju can head to the stables, though, Ping-Cho arrives looking road-weary and vulnerable. She asks to journey with the trade caravan if it is bound for Samarkand. Kuiju is non-committal until Ping-Cho produces money to pay for her passage. Suddenly, the lies come thick and fast: He is certain his "trade caravan" is headed inland and takes the offered money to give to the "commander of the caravan". He goes off to "make arrangements", leaving Ping-Cho alone in the courtyard.

It is not long before she begins to fear she has been cheated and seeks out Wang-Lo. He confirms that she has been robbed, but is more concerned with the fact that Ping-Cho has run away from Marco Polo's caravan. Ping-Cho breaks down, her money gone and her last chance of getting home dashed.

Just then, Ian arrives at the way-station and Ping-Cho rushes to him, crying. She admits she ran away because of the key and because she does not want to marry some old man she's never met. She begs not to be taken back, but Ian has promised. He must return her to Marco.

Ian learns from Wang-Lo that Ping-Cho has been robbed, but he is unsure of the bandit's identity. A short description leads him to Kuiju and he realizes that he too may have been duped. When another man joins him - with the true authorization to take the TARDIS - everyone realizes what has happened. The TARDIS is not on its way to Shang-Tu. It has been stolen.

“I hope all is well and Ian has found Ping-Cho. Our progress towards Shang-Tu continues to be excellent and we are now beyond the Great Wall, spending the fourth night of our journey at an inn about fifty miles from the summer palace.”

Marco's decision to allow Ian to search for Ping-Cho still rankles Tegana. He wants to go back after Ian, but Marco refuses, saying he trusts Ian. Besides, how can Marco go to the Khan without Tegana? His absence could ignite another war with Noghai. Tegana continues, suggesting that Ian didn't even go back to get Ping-Cho but actually went to recover the TARDIS. He doesn't even want Ping-Cho to be found and this has simply been a trick to escape. Marco refuses to believe this, certain that Ian is honourable. Tegana repeats the possibility that without the magic caravan, Marco will never be allowed to return home. Still he is not convinced.

However, Susan and Barbara appear at this moment, laughing and giggling. They seem far too happy seeing as their friend has not yet returned, and may never return. Tegana seizes on this opportunity, implying that they are happy that Ping-Cho has escaped her arranged marriage. The warlord turns to them and speaks idly of Ian's progress in finding Ping-Cho. As expected, Susan blurts out that she hopes Ian doesn't find her so that she can escape this marriage. At a prompting from Tegana, Barbara agrees.

Marco listens to this with growing horror. Barbara can't quite understand what all this means, but she admits that Ian, too, is opposed to Ping-Cho's arranged marriage, despite Kublai Khan's blessing on it. That is all he needs to hear - Marco gives Tegana leave to depart, to find Ian and Ping-Cho and bring them to the Khan's palace in Shang-Tu. Only after Tegana has left does Marco reveal that he has lost his trust in Ian.

Ian and Ping-Cho have scoured Cheng-Ting for the TARDIS and for the one-eyed bandit who stole it. There is no sign. Ian figures the ship must have been taken out of the city by some old and untravelled road as no one has seen it. Ping-Cho suggests the Karakorum road. The city has lost its eminence since the Khan built his palace at Peking, but it is still a place of importance to some Mongols and its road is only lightly travelled. Ian is convinced that is where the ship is and he and Ping-Cho must find it.

At long last, Marco Polo and his caravan - minus a few key members - has reached Kublai Khan's summer palace at Shang-Tu. The palace is large and opulent, with pavilions, pagodas, and expansive grounds. Marco, the Doctor, Susan, and Barbara are led to the throne room by the Khan's Vizier. The Khan will attend them presently. The Doctor is puzzled at the Chinese architecture favoured by the Mongol Kublai Khan, but Susan and Barbara are fascinated by the palace's shining beauty.

The Doctor's natural crankiness has been exacerbated by all of the aches and pains his aged body has incurred due to five days of hard riding on horseback. He is sore and ill-tempered, not really interested in Marco's description of the Khan's land holdings here. He is even more distressed to find out that he must kneel and touch his forehead to the floor when the great Khan appears. Even though refusing to show the proper obeisance could cost him his life, the Doctor refuses to kowtow. Soon, though, a gong sounds, indicating that the Khan's arrival is imminent. All of the others get to their knees. The Doctor at last tries to kneel, but a fierce pain shoots up his spine. He groans in agony as he kneels. Bending any further is impossible. His groans of pain continue.

The mighty Kublai Khan reaches the throne room slowly and painfully. He suffers from gout and his foot is badly swollen. The Khan is an old man with a long silver beard, walking with a cane and a limp. His groans of pain rival those of the Doctor. At first, the Khan thinks someone is mocking his pains, but he quickly sees something of a kindred spirit in the Doctor and bids them all to rise. He welcomes them all to the court. Marco tries to apologize for the Doctor's seeming impertinence. The Khan only seems upset that the Doctor isn't a true physician.

The Khan inquires as to Tegana's whereabouts and Marco tells him the story. His absence worries the Khan as Noghai's army is now encamped at Karakorum, uncomfortably close. Marco is shocked, knowing that Noghai was as far away as Samarkand when Tegana joined his caravan. The Khan is very anxious to hear Tegana's explanation of Noghai's conduct.

However, the Khan is not inclined to wait here for Tegana to return as he plans to leave tomorrow for Peking. Susan wishes to wait for Ping-Cho, but the Khan will not wait. All of them must go together. The Doctor is not at all interested in mounting another horse, but the Khan offers him the chance to ride with him in his carriage of state, and the Doctor is only too delighted.

Suddenly, the Khan is struck by a sharp jolt of pain and the audience is at an end. He suggests that the Doctor join him in the healing waters here and the two old men toddle off together arm in arm, much to Susan's amusement. Barbara tries to talk Marco into staying here a while, to wait for Ian and Ping-Cho, but that is out of the question. Marco is sure Tegana will find them. Barbara is worried what might happen if he does.

On the road to Karakorum, Ian and Ping-Cho have found Kuiju. He has stopped to rest by the roadside. He is clearly waiting for someone, and has even lit a fire. Ping-Cho recognizes him instantly, and the TARDIS is safely tied to a wagon nearby. Ian tries to sneak up on the bandit, but he is ready for trouble and quickly draws his dagger. Ping-Cho bursts from hiding and startles Kuiju. Ian knocks the dagger from his hand and picks it up quickly. Ian grabs the man and holds the dagger against his throat. Kuiju is forced to admit that he stole Ping-Cho's money and to give it back. Ian presses the dagger even closer and forces Kuiju to admit who paid him to steal the TARDIS: Tegana.

At that moment, Tegana appears with his sword drawn. He grabs Ping-Cho and she screams. Ian threatens Kuiju's life to get Tegana to back down, but Tegana cares nothing for the bandit's life. He urges Ian to do what he will.

Ian asks if the mighty Tegana is willing to kill a child to achieve his aims and he says he will if she is in the way. Ian asks if Kublai Khan is also in his way. His only reply is that his Lord Noghai will rule the world once he possesses the TARDIS. He, too, has sorcerers to make it work. With that, he has had enough of this game. He moves his sword as if to strike Ping-Cho. At that moment, a group of warriors surrounds them, led by the Khan's courier Ling-Tau. He orders the two combatants disarmed. Now freed, Kuiju tries to flee. Once of Ling-Tau's men, overzealous in his duty, catches him and kills him. Ling-Tau is of course displeased.

Tegana regains himself quickly, trying to assert his authority as a Mongol warlord. He tells Ling-Tau that Ian and Ping-Cho were trying to steal the Khan's property - the TARDIS. Ian refutes the charge and Ping-Cho asserts that Tegana is the thief. Uncertain who is telling the truth, Ling-Tau opts to take them all to Kublai Khan. However, he tells them they must ride to Peking as the Khan has left Shang-Tu.

In the throne room of Kublai Khan's opulent Peking palace, the Doctor and the Khan are comfortably ensconced at a gaming table, their physical ailments forgotten for the moment. The Doctor seems to be doing very well indeed, winning yet another game from the amiable Khan. In fact, the Doctor reads off the tally of his winnings: 35 elephants with all of their trappings, 4000 white stallions, 25 tigers, the sacred tooth of the Buddha (a gift from Marco), and all the commerce from Burma for one year.

While the Khan thinks this is not too bad, he reacts guiltily when he hears the Empress approaching. He urges the Doctor to hide the tally sheet. The Empress inquires idly about the state of the game, and both the Doctor and the Khan avoid telling her the truth. She seems suspicious and reminds her husband not to place wagers on the games as it always affects his gout. She departs, leaving the Khan to try and hide the renewed pain in his foot.

The Doctor comments favourably on the "charming" Empress, but the Khan somehow wishes he had the strength to stand up to her more forcefully. He fears she will be furious when she learns all that he has lost. The Doctor tries to cancel the debts, but the Khan says he is honour-bound to pay them. The Doctor then makes a counter-offer: all of his winnings in exchange for the TARDIS. At first, the Khan does not accept this. He hasn't even seen the gift yet and is being asked to give it away. But the Doctor persists, and in the end the Khan agrees and the game commences.

They are interrupted almost immediately by the arrival of Marco Polo. Irritated, the Khan agrees to hear him. Marco reports that Tegana has reached Peking and should be at the palace presently. The Khan thanks Marco and then delicately informs him of the wager he has made with the Doctor. As expected, Marco is distraught, sure that his chances for returning home have been dashed forever. But he dares not protest and leaves the room dejected. The Doctor rolls the dice to begin the final game.

Elsewhere in the palace, Marco finds Barbara and Susan. Grimly, he tells them of the backgammon game and of the wager. Susan is delighted, but Marco believes this just another of the Doctor's tricks. Barbara sees the Doctor's strategy: Marco has given his gift and should get approval to go home and the Doctor will win the TARDIS back fairly so they can go home too. Susan is sad, though, because Ping-Cho still is unable to go home. Marco reports that her future husband has already reached the palace, where a banquet will be held in his honour tonight. The wedding is set for tomorrow morning.

Ling-Tau enters, finding the group in conversation. He looks grim as well, reporting that Ian and Ping-Cho are being held under guard, accused of stealing the Khan's property. Susan tries to reason that the Doctor will soon win it back from the Khan, but that will not excuse Ian and Ping-Cho. On top of that, the Doctor soon joins them, saying nothing. The downcast expression on his face speaks clearly enough - he has: lost his game. Aware of the irony, the Doctor chuckles ruefully to himself as he shows off his consolation prize - a piece of the Khan's paper currency.

Later, down in the palace's cell-block, Marco listens to Ian's account of what happened on the Karakorum road. He is intrigued by the possibility that Tegana may have declared the Khan his enemy, but Ling-Tau cannot confirm this and Ping-Cho's word will not be admitted in any trial. It seems her fiance has asked the Khan to excuse her in exchange for him taking her away from Peking after the wedding tomorrow.

At long last, the TARDIS has been delivered to Kublai Khan. He inspects it with Tegana as it sits in the throne room. Tegana uses the opportunity to continue his influence of evil. He lets slip that the Doctor has tried several times to steal the magic caravan away from Marco, some also involving Ping-Cho. The Khan is appalled that Marco did not mention this and Tegana points out that neither Marco nor the strangers are Mongols. Is it not natural that Marco would protect them?

Marco joins them in the throne room and the Khan is quick to question him about the attempt to steal the TARDIS. It is clear that Marco regrets his actions in taking the caravan in the first place. His earlier doubts kept him from being more forceful, and now he admits that taking the strangers' property was wrong. He admits that he only wanted to use the gift to get permission to go home. The Khan is angered by Marco's desire to bribe him and claims the TARDIS as his rightful winnings at backgammon. He tells Marco to bring him the key after the banquet tonight. Marco tries to make the Khan understand his actions, but he dismisses Marco saying he is distressed and angry. The Khan also sends Tegana off, telling him they will meet to discuss the terms of peace with Noghai after the banquet as well. But he tells Tegana that he will be on his guard then due to the warlord's formidable powers of persuasion.

Later that night, though, everything has changed. The Khan and the Empress are alone in the throne room, awaiting Ping-Cho. The Empress is crying and the Khan is lamenting a tragedy. Ping-Cho arrives and the Khan tries to break the news gently: her elderly husband-to-be drank a youth elixir composed of quicksilver and sulphur in order to be "worthy" of her love. He died there at the table. The Empress weeps anew, anxious to share in Ping-Cho's grief. But the young woman does not grieve over him any more than she would over any other elderly stranger who died. She did not know or love him.

The Khan seems to understand. With her marriage contract now terminated, Ping-Cho is free. He offers her the opportunity to return home or to stay here as a favourite at court. She hesitates, unsure which to choose. However, when Ling-Tau enters with Marco a moment later, her decision seems to be made in the gaze she shares with the handsome courier. She will stay. This seems to please the Khan and he asks Ling-Tau to escort Ping-Cho back to her quarters, her new home. Before she goes, he asks her opinion of the strangers. She says they are and always will be her friends. Then she is off.

The Khan tells Marco harshly that he has lost the Khan's trust. He is saddened by Marco's deceit. If he does not prove his honesty and truthfulness again, he will be banished from the court forever. He asks for the TARDIS key, but Marco urges him to have the Doctor present when he tries to open the caravan. Marco is sent to fetch the Doctor, during which time, the Khan will meet with Tegana.

As Marco leaves, he runs into Tegana just outside. He is now certain of the warlord's treachery and can see his evil influence over the Khan. He has underestimated Tegana. Tegana simply smiles and tells him that he overestimated himself.

In the cell-block, The Doctor, Susan, and Barbara visit Ian in his cell. They are still trying to find a way out of this dilemma, but due to Tegana's influence, the Khan will not even grant them an audience. They know that Tegana is behind all of this. He has wanted the TARDIS for his master, that much they know, but the Doctor is convinced that his plans go back long before they arrived. Soon Barbara recalls what the Khan said about Noghai being on the march with his army and things start to fall into place for the Doctor.

He realizes that Tegana was trying to delay Marco's caravan in any way he could - including the fake "bandit" attack - in order to give Noghai's army more time to march. But the Khan's army is more powerful than Noghai's and the only way Noghai could hope to win is to first destabilise that army. Kill its leader and even the greatest army is vulnerable. It's happened numerous times in history. They realize that Tegana's plan has always been to assassinate Kublai Khan and they must stop him.

The Doctor calls to the guard to let them out. As soon as he opens the door, the Doctor trips him with his walking stick and all four bolt for the throne room. Unfortunately, they run into Marco on the way. He does not believe their story and takes them all prisoner. However, Ling-Tau finds them, panic in his voice as he reports Noghai's army is marching toward Peking right now. This is all Marco needs to hear. He leaves the others and races to the throne room.

In the throne room, the pretense of peace has reached its end. Tegana listens to the Khan tell him that all who rise against him must be humbled, and then draws his sword. He will now uphold his promise to Noghai to "make an end" of this matter. An attendant engages Tegana to protect his master, but Tegana despatches him quickly and brutally.

Just as Tegana steps forward to strike the defenseless Khan, Marco bursts into the room. The two men, now mortal enemies evenly matched, engage in a sword fight which will end their conflict. Tegana fights hard, but his fury is no match for Marco's strength and skill. Before long, Tegana is cornered and Marco knocks the sword from his hand.

Ling-Tau arrives with a group of warriors, followed by the Doctor, his companions, and Ping-Cho. The Khan, enraged by the deceit of both Tegana and his master, orders Tegana's death. But before the sentence can be carried out, Tegana grabs the sword of a nearby guard and kills himself. He has taken the only honourable path left for him.

Marco orders the body taken away. In the chaos that follows, he hands the Doctor both TARDIS keys and urges him to leave immediately. The travellers hesitate, but Marco tells them to hurry. The Doctor thanks him and Susan says a rushed goodbye to Ping-Cho, then they all hurry inside. The Doctor closes the doors and dematerializes a moment later.

Marco and the Khan watch as the magic caravan disappears. Marco apologizes for giving it back to them, but the Khan does not care. He is certain the Doctor would have won it back eventually. He marvels at the machine, telling Marco he will amaze his friends with this story when he returns to Venice. Marco realizes he has been given leave to go home and is thrilled. But he's not sure he'll mention this particular miracle. Who would believe it?