The Black Guardian demands an explanation, but the Doctor argues that all he’s been doing is clearing up everyone else’s messes. The Guardian is angry that the Doctor is working for the Grace, as it suggests that he himself couldn’t be trusted to help. Amy has been frozen in time, along with everything else, and the Guardian orders the Doctor to hand over his segments of the Key to Time. The Doctor refuses to co-operate and the Guardian threatens to let the Martian spaceship continue plunging into the sun, but the Doctor finds it curious that he doesn’t just snap his fingers and take the pieces himself.

The Guardian’s deep booming voice changes to a more normal tone and he admits that because the segments are already outside time, they’re also beyond his reach. The Guardian is tired of the stereotypical negative image he normally projects and says there’s more to him than just that. He’s responsible for 50% of reality and he resents the fact that people think the White Guardian is nicer because he puts on a doddery old man act. He says the White Guardian represents law, bureaucracy and red-tape, whereas he is the opposite - but he argues that chaos isn’t bad in itself and it’s the variation that makes life interesting and adds colour. The Doctor remembers what he did to Turlough and refuses to be taken in by this new “cheeky chappy” façade, but the Guardian expects a bit of gratitude for saving his life on the spaceship.

The Doctor realises the Guardian wants the segments so he can draw on their energy. The Black and White Guardians are both looking for the same thing - the two remaining segments. They already know that the sixth segment is in an organic configuration, but if they find that segment piece before the fifth, the removal of the host lifeform from the continuum with cause a temporal quake. Both Guardians have had legions of minions at work for millennia, but they haven’t found a thing yet and their power is getting so weak they’re finding it difficult to command large groups. The Black Guardian has therefore decided to limit his sphere of influence to just one - the Doctor. Amy suddenly comes to life and warns the Doctor not to listen to the Guardian’s lies, which is further evidence that his powers are fading fast. The spaceship also starts moving again and resumes its dive into the sun. The Doctor urges the Guardian to get them out of here quickly…

There’s a huge flash and the Doctor, Amy and the Guardian all find themselves literally flying through the air. The Guardian says he wants to show them something. He points to the River Nile below and tells them this is 9th century Sudan, the time of the Caliph al-Mutawakkil. Amy is getting a strange tingle, but it’s not like before and it’s not enough to suggest the next segment is anywhere close. The Guardian explains that he’s tried everywhere and there’s no trace of the segment being anywhere whatsoever. To illustrate the point he takes the Doctor and Amy to a location three miles beneath the crust of the planet Murphax VII where a volcanic eruption is imminent. The planet is orbiting the star of Perseus and if it wasn’t for the Guardian’s force field, the extreme heat would kill them. Amy feels as though the segment should be here, but she knows it isn’t.

The Guardian says that after a few more millennia he stumbled across the planet Alpha III. He takes them to a dead forest which will soon find itself under a mile of ice for ten thousand years. Amy still can’t sense any of the segments, but the Guardian says that in all the time he was searching, these locations were the prime candidates. He admits defeat and says he can’t offer any advice as to where the segments could be. They could be disguised as anything - a grain of sand, a leopard’s tooth, a blob of molten lava or even an atom of snot. He searched throughout time and can’t find them. They’re out of options and if they fail in their mission, it’ll mean the end of time.

The Doctor suggests they try something completely at random. He says the Guardian has been absolutely methodical in his methods, so the only thing left to do is to search in a manner that’s heedless of any sense and dictated only by chaos. It requires a leap of faith, so the Guardian agrees to let them find their own destination and says he’ll track them down and join them once they’ve gone. He can already feel the onset of entropy, so every second counts. He uses the last of his power to send the Doctor and Amy on their way, with no idea where they will end up…

The Doctor comes crashing to earth and finds himself in the middle of a desert. Amy is nowhere to be seen. His arrival has been seen by a group of men from a nearby camel caravan and they immediately mistake him for a bandit. The leader of the men, Hasan, mounts his camel and charges towards his prey with his lance raised, ready to kill the bandit. The Doctor insists that he‘s merely a lost traveller, but the men don‘t believe him and are convinced he has criminal brethren waiting nearby. When it becomes clear that the Doctor isn’t even armed and appears to have just fallen out of the sky, the men back down, but Hasan remains cautious and grants him permission to continue living as his slave. When the Doctor tries to explain how he got there, Hasan mistakes him for a story-teller and assumes he’s heading for the city of Dunqulah.

In the city of Dunqulah itself, the Vizier asks Lord Cassim Ali Baba if he’s feeling troubled by anything as he heard him call out for a doctor, but Cassim says he was simply dreaming and had suddenly been struck by a memory from long ago. For tonight’s entertainment, the Vizier has arranged some performing animals, acrobats and story-tellers, but Cassim finds the repetitive acts are getting boring now. He finally despairs when the Vizier tells him he’s found a troupe of professional farters who perform tricks with human wind.

The Vizier goes to the kitchens and tells the slave Nisrin that Lord Cassim has approved 40 entertainments and she needs to prepare enough food for everyone. Suddenly they hear a woman scream outside, as if falling from a great height, and they see Amy thrashing around in the courtyard fountain. Nisrin doesn’t recognise her, but the Vizier assumes she’s one of the new slave girls and tells Nisrin to teach her some decorum. The woman orders Amy out of the water, but Amy is frantic because she’s lost her satchel. Nisrin takes pity on her when she asks for the Doctor and arranges to get her some dry clothes.

The Doctor is invited to travel to Dunqulah with the caravan, but he’s advised to be wary of the camels as they can bite and spit. The caravan is said to contain forty different kinds of cargo, including food, swords, jewels and even slaves, but the Doctor soon learns that forty isn’t an exact amount but simply a routine definition applied to any amount. Hasan describes his group as the lifeblood of the Caliphate and the Doctor realises he’s come back to the Sudan at the time of the Caliph al-Mutawakkil. This is one of the places where the segment “almost was”, which must be more than just a coincidence.

Nisrin finds Amy standing on the roof and wonders why she’s not downstairs enjoying the entertainment. Amy has completed her tasks for the day and says she was using her free time to look for her satchel. Nisrin assumed she’d come here to look at the stars, but Amy admits that the thought hadn’t occurred to her. Nisrin points out the stars known as Al-Debaran, Al-Rigel and Yad Al-Jawz?. When Amy claims to have visited the stars with the Doctor, Nisrin thinks is ludicrous and calls her a “teller of unlikely tales“. She recounts the story of a man from the tribe of Udra who disappeared one day and when he returned, said he’d been carried off by the Djinn - creatures of subtle fire whose trails are seen in the night skies - and taken to distant worlds, but nobody believed his stories. She says Prince Omar knows the names of all the stars, so she’s been studying the subject herself in the hope that he’ll grow to like her.

Later that night, Lord Cassim slips out of the palace unnoticed and checks in with his alien associate. He asks if they’ve recovered enough gold yet, but the alien says they still need much more. It orders him to get more gold tomorrow, but Cassim says things are getting difficult and they might all be dead by then. The alien ignores his protests and demands that he tries harder.

The camel caravan has stopped for the night and as the crew make a campfire, the Doctor looks up and admires the stars. Hasan tells him it was lucky they met because the Doctor was actually walking away from the city and further into the desert. He says they’re not far from Dunqulah but because the caravan is slow, it’ll take them another day to reach it. He wonders if the city will still be there when they arrive and explains that Lord Cassim is believed to be mad. For two years he’s been hoarding the gold from his mines and refusing to pay his tithe to the Caliph, an action that can only lead to a show of force from the Caliph’s soldiers. The Doctor wonders why Cassim needs all that gold and thinks someone should have a word with him…

Lord Cassim returns to the palace where his son, Prince Omar, rebukes him again for taking leave of his senses and for withholding the gold from the Caliph. Cassim refuses to be questioned on this matter any more and claims that Omar couldn’t begin to understand his reasons. Omar is worried the Caliph will soon send soldiers to remove his father from office, but Cassim points out that this would leave Omar himself in charge. Omar insists that his loyalty is to his father, but he’s worried that nothing will be left for him to inherit but ruins and sand. Omar continues to protest, but Cassim has heard enough and leaves.

The next morning, Hasan wakes the Doctor up and tells him they have a lot of ground to cover today. The Doctor thinks he can see smoke in the distance, but Hasan realises it’s not smoke, but sand. He urgently calls his crew to attention and orders them to run for Dunqulah before the sandstorm hits them as the sand will cut into them like knives and there’s no shelter nearby. The Doctor points out some nearby tents, but Hasan says they belong to the Caliph’s soldiers and he has no wish to be associated with them. The traders flee for their lives, but the Doctor stays behind, urging them to reconsider and take the more sensible option.

Nisrin and Amy are busy washing crates full of clothes. Amy is shocked to learn that Nisrin considers herself to be a slave and is even more surprised when Nisrin reveals that she was taken from her parents as a child, sold to the Caliph and given a new name. Amy wants Nisrin to teach her to be more like her, but Nisrin says she doesn’t need to be taught how to use a mop and bucket. Prince Omar joins them and he‘s instantly taken by Amy’s beauty, but Nisrin apologises for the girl’s behaviour and explains that she’s new to her role. Omar warns them both to get inside as a storm is approaching. Once they’re alone again, Amy senses that Nisrin’s attitude towards her has changed. Nisrin insists that Omar belongs to her (even though Amy points out that it’s actually the other way round) and says she no longer wants to be invisible to the Prince. She plans to become the mother of his child and put her slavery behind her. Amy thinks it would be a good idea for her to try that as well, but Nisrin warns her that if she flirts with Omar again, she’ll introduce her to the Destroyer of Delights!

The execution is interrupted by the arrival of the Legate of the Caliph who insists their prisoner is given a chance to defend himself. The Doctor argues the case for being innocent until proven guilty and is about to make a case for justice and law when he suddenly realises the Legate has been calling him by his name/ When he notices that the guards refer to the Legate as the agent of law, it dawns on the Doctor that his captor is none other than the White Guardian…

Prince Omar greets Hasan who arrives at the palace in a dishevelled state, having been caught up in the sandstorm. The caravan has made it here in one piece, but Hasan reports that he spotted a fully-armed battalion of the Caliph in the desert. While they waited out the storm, Hasan ran here as quickly as he could, leaving his new friend the Doctor behind.

While working close to the palace throne room, Amy listens in on Prince Omar’s conversation with Hasan. Omar reveals that his father has taken leave of his senses, but Hasan warns the Prince against defying him. Omar is torn between his loyalty to his father and his loyalty to the Caliph. Hasan suggests that if Omar finds out where Lord Cassim is hoarding the gold, he might get a better idea of his father’s reasons. Omar decides to follow Cassim the next time he leaves the palace, but Hasan says it would be better to send someone expendable…

The White Guardian accuses the Doctor of being in league with the Black Guardian, but the Doctor insists he was left with little choice as time itself has started disappearing and the entire fourth dimension is collapsing to a point where it will soon be indistinguishable from the first. It occurs to the Doctor that the White Guardian’s powers must be waning too and he‘s stuck on this planet just like the rest of them. If he’s trapped in the form of the Legate, he’ll be experiencing the Universe through human eyes for the first time.

Amy confesses to Nisrin that she secretly listened in on Prince Omar’s conversation with Hasan. Nisrin is furious with her and attempts to punish her, but the beating has no effect on Amy. Omar is attracted by the commotion and Nisrin is forced to explain what‘s going on. Amy admits the truth, but to her surprise Omar simply congratulates her on her skill as a spy and says if it wasn’t for the noise Nisrin was making, he’d never have known they were here. He tells Amy he has need of her special talents and asks her to follow Cassim to find out where he hides his gold. He also tells Nisrin that when he takes over from his father, he’ll want her to take charge of the harem and if she obeys his orders, her daughters will becomes the wives of kings.

The White Guardian admits that he couldn’t find any sign of the fifth segment either, so when he found the TARDIS drifting in space nearby, he followed the Doctor to this location. As the symbol of law and order, he searched for the ultimate law on this world and discovered it was the Caliphate of Baghdad. He then inserted himself into the local time stream and was disappointed to find he’d become nothing more than the hatchet man for a weedy despot. He’s been waiting here ever since for the Doctor to arrive. The Doctor realises the forces of time are all converging at this point in history, drawing them all here - but unless he finds the segment, the Guardian plans to continue with his plan to wipe out the ruling family of Dunqulah. A guard informs him that the storm has stopped, so they prepare to move out. It’ll take them another day to reach Cassim‘s palace, so the Doctor asks for a camel so he can reach Dunqulah ahead of them. If he can bring Cassim round and persuade him to pay his taxes, they can still avert any bloodshed. The Guardian gives him permission to try, but warns that he’ll be close behind, ready to unleash the Destroyer of Delights should he fail in his mission…

Nisrin insists on accompanying Amy on the trail of Lord Cassim, although Amy suspects it’s just so she can share in some of the glory should her mission be successful. They cover themselves in black robes, partly to hide themselves in the dark, but also to protect themselves against the ravages of the sand. They watch as Cassim and his guards pass by, carrying crates filled with gold. The Vizier expresses concerns to Lord Cassim about the army rumoured to be approaching the city. He suggests it might be better to hand the gold over to the Legate, but Cassim questions his loyalty and tells him the gold has a higher purpose. Amy and Nisrin follow from a safe distance as Cassim leads his guards deep into the desert, then when they reach their intended destination Cassim orders everyone else to return to the palace while he stays behind with the gold.

Amy notices that the sand dune they’re hiding behind is unusually curved and just beneath them she can feel metal. She sees Cassim clearly for the first time and instantly recognises him as the Black Guardian. She tells Nisrin that he was the man who sent her here in the first place and insists that he’s the enemy of all that is good, but Nisrin warns her about the dangers of speaking about their masters in such a way. Amy spots an airlock at the base of the dune and they listen as Cassim gives a password. The door slides open to reveal what Nisrin thinks is a magical cave, but Amy knows is actually a spaceship buried under the sand. Two huge robots emerge and carry the crates of gold into the ship, then Amy suggests she and Nisrin follow them inside too. She’s convinced the robots are very basic in design and won’t be “set” to look for intruders. To prove this, she approaches the robots and is delighted when they don’t react to her presence. She calls to Nisrin and together they move into the airlock…

Inside the spaceship, the alien supervises as the robots check the weight of the gold, but Cassim is frustrated when it turns out they still don’t have enough yet. The alien tells him it needs the weight of another human in gold, but Cassim knows time is running out. He knows the alien is keen to leave this world too, but he’s worried the Legate of the Caliph is only a day away. He points out that they’ve spent two years mining and there’s no more gold in the kingdom that he can offer. To help Cassim stop the Legate‘s army, the alien produces a device and tells him it will solve all their problems once it’s handed to the Legate.

Nisrin is still at the airlock, struggling to stop the door from closing and trapping Amy inside the “cave”. Eventually it becomes too much for her and she finds herself outside the ship, banging on the door to get Amy’s attention. The computer control asks her for the password, but fortunately she remembers the words Cassim used earlier and the door slides open again. She sees Amy racing towards her, with a figure not far behind her, hidden in the shadows, firing a magical weapon of fire. The two women escape from the ship and take cover behind some nearby rocks. Amy assures Nisrin that the alien can’t follow them as it isn’t wearing an environment suit to protect it against the atmosphere. She’s delighted to get her satchel back and thanks her friend for saving her life, but Nisrin admits that she’s unhappy serving in the palace and has often prayed for adventure. Now they know where the stockpile of gold is hidden, Nisrin thinks Prince Omar will be delighted with them, but this worries Amy because what they‘ve discovered here is beyond the understanding of anyone from this time.

The Doctor arrives at the palace on the back of a camel and is greeted with some surprise by Hasan. The trader is cautious when the Doctor reveals that he sought refuge with the Caliph’s army and he wonders why the Legate didn’t simply have him killed. The Doctor says he’s committed to doing whatever he can to prevent people from killing each other, but Hasan suspects he now works for the enemies of Cassim. Fortunately Hasan now considers himself to be an enemy of Cassim too and offers to take the Doctor to meet Prince Omar. Later, Hasan has a chance encounter with Amy, who’s delighted to learn that her friend is still alive and that he didn’t abandon her as she’d previously thought.

Another surprise visitor at the palace is the alien from the spaceship. A guard tries to bar the new arrival from entering, but the alien announces that he too is a servant of Lord Cassim and has come to see him on important business. The guard still refuses to let him pass, so the alien produces its laser weapon and threatens to send him to the Destroyer of Delights…

The Doctor introduces himself to Prince Omar as a messenger from the Legate of the Caliph. Omar admits that the people of Dunqulah are torn in their allegiance as they know that Cassim is defying the very man who appointed him their leader. Omar himself now believes his father to be mad and is preparing to “divorce” him. He tells the Doctor that the gold is being kept in a cave in the desert that’s guarded by a fierce creature…

Lord Cassim agrees to a meeting with the Doctor, but just as the Doctor is about to launch into a tirade about the despot’s behaviour, he realises the man is none other than the Black Guardian. The Guardian recalls that the last time he saw the Doctor, through the dimensional portal, he was in the desert, fleeing from a warrior on a camel next to a caravan of traders. The Doctor is angry that the Guardian didn’t intervene at the time and save him a lot of trouble, but although this incident only happened yesterday, the Guardian is convinced it was decades ago. It’s clear that as time recedes, so does the Guardian’s mastery of it. He reveals that he was brought to Earth aboard a spaceship, which is now powerless in the desert as they‘ve been unable to repair the warp manifold. Gold is the only suitable resource available in this medieval society and he nearly has enough now. When the Doctor reveals that the White Guardian is here too, in the form of the Legate of the Caliph, the Black Guardian warns that there‘ll be a fight. The Doctor knows that all eternity now depends on the outcome of these events - they’re not really fighting over tax in a desert kingdom, they’re fighting for victory over all time. One of them will perish, but with reality winding down, the other will discover he is the master of nothing but ruins.

The Vizier arrives with news that a blue man has arrived from out of the desert, so the Black Guardian makes his excuses and leaves. The Doctor explores the palace and finds Nisrin struggling to carry a huge stone jar. He offers to help, but she says it would be wrong for him to come to her aid. Eventually he’s forced to order her to let him help and they discuss the poor treatment this so-called civilised society dishes out to slaves who fail in their duties. When she realises who he is, Nisrin reveals that his friend Amy is here too, but it‘s when she describes the “metal servants” out in the desert that his curiosity is stirred. Amy returns and as the Doctor greets her warmly, she proudly shows him her satchel containing the other segments. They discuss the blue alien who the women encountered in the desert but the Doctor explains that in the language of the people here, blue actually means black.

The Guardian is furious with the alien - he’s been hiding out in the desert unnoticed for forty years and now he’s come walking up to the front gates where everyone can see him! The alien explains that they’ve already been discovered by a slave who followed him from the palace and has seen the treasures aboard his ship. He also reveals that the slave stole one of his own personal belongings, a handbag with an enclosed supra-dimensional interior. The alien insists that if they don’t leave this world tonight, they’ll never get away. Just then, the alien sees Amy and points out that she was the one who stole his treasure…

The Doctor is amazed to discover the alien really is blue after all. He recognises the man as one of the Djinn, a race of alien collectors who’ll buy and sell anything and go from world to world in search of profit. The alien orders him to stand aside so that he can kill Amy, but the Doctor insists she’s just an innocent girl. Lord Cassim intervenes and orders the Djinni to put away his weapon and says they have far bigger problems to sort out than a stolen bag. Instead, Cassim hands Amy a jewelled necklace and tells her to ride out with his son to the approaching army and present it to the Legate of Caliph as a token of his friendship. The Doctor is delighted with the Black Guardian when he declares there should be an end to the animosity between his kingdom and the Caliph. As Amy leaves, Cassim tells the Djinni they must prepare a welcome for their guests. With the problem apparently sorted, the Doctor is for once at a loss to know what to do next, so Cassim asks him to go and find the Vizier.

The Vizier and Hasan are praying for absolution, having heard from the guards that a blue creature - undoubtedly the Destroyer of Delights - has arrived at the palace. The Doctor joins them and tries to persuade them not to worry as everything will be fine. He explains that Amy and Omar are making their way right now to the army of the Legate and there’ll be no war after all. However, when the Vizier points out that the Legate has already visited all the other cities to collect their taxes and left Dunqulah till last, the Doctor begins to get suspicious. This means the approaching caravan will be filled with gold - the very substance that the Black Guardian and the Djinni need to complete their plans!

The Doctor immediately confronts Lord Cassim and accuses him of planning an attack on the Legate’s caravan to steal their gold. He realises the Djinn compress the mass of substances like heavy gold to fold it in upon itself at the molecular level to make their warp manifold. He warns that their actions will start a war and plunge the entire region into chaos, but the Black Guardian points out that this is his purpose in life. The Doctor remembers the red jewel that Captain Wrack placed in Tegan’s tiara as a tracking device and realises there must be something similar on the jewelled necklace the Guardian gave Amy. The Djinni will have weapons aboard his vessel and will be able to use the necklace to aim them right into the Legate’s caravan with pinpoint accuracy. The Doctor reminds him that Prince Omar is with Amy, but Cassim is quite content for his son to die a hero. At least it means he won’t have to have him killed for plotting against him. The Doctor says he’ll put a stop to this, but Cassim suddenly calls for his guards and the room is filled with soldiers who take the Doctor prisoner and throw him into a cell. His actions make the Vizier wonder whether his loyalties are to the right man…

The Doctor is surprised when Nisrin visits him in the cell. She told the guard she was here on official duties and she knew he’d never suspect a slave would dare betray her master. She’s decided that her true loyalties are to Omar and she hopes that by rescuing the Doctor, she’ll help to save the Prince‘s life. When the Doctor tells her about the weapons on the Djinn ship and the tracker on the necklace Amy is carrying, Nisrin reveals that she’s already been inside the alien spaceship and says she can try to distract the Djinni from firing his weapon.

As they cross the desert, Prince Omar strikes up a conversation with Amy and assures her he’s not a monster, but she thinks he should care more for the thoughts of Nisrin too as it‘s obvious she loves him. They reach the army caravan and Prince Omar introduces himself to the Legate, then Amy steps forward to present him with the gift from the Lord Cassim. The Legate is impressed and hopes this means there won’t be any trouble now. However, the meeting is interrupted by the sight of the Doctor in the distance racing towards them. The White Guardian notes that the Doctor was true to his word and says he deserves a share of the credit for averting war.

The air above the Legate’s caravan is filled with the sound of thunder, but Amy recognises it immediately and realises someone is shooting at them. The Doctor calls out to them and warns them they’re in danger…

The Black Guardian is keen to know whether the attack was a success, but the Djinni explains that because his ship has been stuck in the sand for decades, there’s been some damage to the weapons’ sights. The Guardian is furious and orders the alien to fire again…

The Doctor finally arrives at the caravan and calls to everyone to take cover as there’s another attack on its way. They listen to the roar of gunfire as the ground explodes around them. The Doctor demands the necklace from Amy, but the Legate is reluctant to hand over his gift until he learns that it’s acting as a lure for the Destroyer of Delights. The Doctor says it’s the only way the alien can find their location from such a long distance and he suggests using one of the army catapults to get it as far away as possible.

The bombardments are still off-target so the Djinni prepares to re-set his equipment again, but then Nisrin emerges from hiding and points an energy weapon at him. She rebukes the Black Guardian for being so readily prepared to kill his own son, the Prince. The Guardian dismisses Omar as nothing more than an accidental bi-product of his imprisonment here. While they’re both distracted, the Djinni prepares to fire off another barrage, so Nisrin turns her weapon on him…

Many of the Legate’s men have been killed by the Djinni attack. The Doctor arranges for the necklace to be placed into the nearest catapult, then Omar pulls the lever and the tracer is sent high up into the sky. It eventually falls far from the caravan and the Doctor says they can only hope the distance is enough to save them.

The Djinni warns Nisrin that she’s holding a deadly weapon in a room full of electrical cables and it could kill her just as easily as them, but she says she doesn’t care and is prepared to destroy all of them if she has to. The fire alarm goes off and Nisrin is distracted by the computerised voice. The Djinni pulls a live cable from the wall and thrusts it towards the slave girl, telling her this is what lightning feels like. She screams in agony and falls to the floor. The alien says she’s only stunned as the shock wasn’t enough to kill her, but he summons a robot to have her thrown outside the ship. When the Djinni returns, the control panel indicates that his original co-ordinates for the target were wrong by a considerable distance, so he adjusts the equipment to compensate.

The Doctor and the others hear the sound of the next attack approaching and Amy wonders if the sensation she’s feeling is fear. However, the group is delighted when the bombardment lands far away from them.

The Djinni checks his computer and reports that there’s no longer any signal coming from the tracer, which means it must have been destroyed. The Black Guardian is delighted and prepares to send his guards into the desert to retrieve the gold, but the Djinni no longer wishes to make use of human servants and wants to send his own robots instead.

Now that the attacks have stopped, Amy confirms that all the Legate’s soldiers are dead and she, the Doctor, Prince Omar and the Legate himself are the only survivors. Omar tries to assure the Legate that he had nothing to do with the attack and points out that his father was willing to let him die too. The Doctor suggests they head for the city, but then they see a group of robots approaching them. Amy recognises them from the alien ship and the Doctor realises they must be coming for the gold. Although they’re convinced the robots will ignore them and concentrate purely on the gold, it seems they’ve been re-programmed to collect any survivors too. The metal creatures grab hold of them and lift them off the ground. The others panic, but the Doctor tells them not to worry as he wanted to get inside the Djinni’s vessel anyway. In fact, being carried there by a robot is more comfortable than sitting on a camel, so they might as well just relax and enjoy the ride.

At the palace of Dunqulah, Hasan informs the Vizier that the thunder has stopped and as the world hasn’t ended as they feared, it seems that their prayers have been answered.

When the robots arrive outside the crashed spaceship, it confirms the Doctor’s suspicions that it’s a long-range trading orbiter. The Black Guardian greets them and approaches his opponent, the White Guardian, with interest. The group is taken inside, but the Black Guardian orders Prince Omar to be left outside with all the rubbish while the others are told they’re going to help him search for the missing segment of the Key to Time. Outside the ship, Omar bangs on the door and insists on being let inside. He finds Nisrin lying nearby in the sand and she tells him she did her best to save him from the barrage. She offers to open the airlock for him and he realises for the first time that she has hidden talents too. Omar decides it’s time to restore order to his kingdom, whether his father likes it or not…

The Black Guardian leads his prisoners into the vault of the Djinn ship. The Doctor is impressed by the treasure trove and says the ship must have been very active in the surrounding star system. The robots arrive carrying the gold from the Legate’s caravan. The White Guardian objects to the theft of the Caliphate’s taxes, but the Black Guardian has heard enough and stuns him with a shot from his weapon. The Djinni takes some readings and announces that they now have the correct mass. The Black Guardian is delighted and tells his ally to begin compressing the gold.

Elsewhere in the ship, Nisrin leads Omar to an observation window overlooking the vault. Omar is amazed to see so much gold, but Nisrin is concerned that it’s glowing as if on fire. Then, to Omar’s horror, the gold starts shrinking…

When the process is finished, Amy asks where all the gold went, but the Doctor assures her it’s still there albeit in a different form. The Black Guardian picks up a small shard of metal and explains that it’s the perfect catalyst for a spacecraft’s warp manifold. The Djinni suggests they install it immediately and prepare to leave this world. They leave the vault and seal the prisoners inside. Amy is starting to tingle intensely and that can mean only one thing…the shard of heavy gold is the fifth segment of the Key to Time! She can’t understand why the Black Guardian didn’t realise it when he picked it up, but the Doctor remembers the Guardian brought them here because this is where he thought he might find the segment - but until now the segment was beyond time. Everyone they were taken by the Guardian was a place where the creation of a segment was imminent, but it never occurred to him that he had to actually be there to make it happen.

The Black Guardian is surprised that the shard of heavy gold isn’t too heavy for him to lift and assumes it must be because there was a change at the atomic level. Suddenly Omar and Nisrin emerge from hiding and, brandishing a sword, the Prince accuses him of destroying his birthright and betraying their people. He demand they hand over the gold, but then the Djinni produces a weapon and fires straight at Omar. For some reason it doesn’t work, so Omar attacks the blue alien and hacks him to pieces with his sword.

The Guardian is furious with his son - the Djinni was the pilot of this ship and now there’s no chance of him getting off this planet. Omar is only interested in retrieving his inheritance and although the gold has clearly gone, he sets off in search of a suitable replacement. The Doctor and Amy arrive, with the recently revived White Guardian and discover what Omar has done. The two Guardians start arguing, but the Doctor intervenes and reminds them they have more important things to deal with. To Amy’s delight, Omar hands her the gold shard, then Nisrin suggests they search among the treasures for something they can use to assuage the Caliph. Suddenly the ship’s computer starts issuing a sequence of numbers and the Doctor realises it’s a countdown. He says the Djinn are notoriously covetous and if one of them dies, he takes his whole ship with him. The computer has clearly started a self-destruct sequence!

Omar and Nisrin discover a room filled with enough silver to pay forty Caliphs. They can hear the countdown too, but Omar believes the “magic cave” is alive and is trying to warn its master. Nisrin wonders whether the cave will try to avenge him…

The Doctor leads the others to the airlock, but they discover the Djinni must have overridden the codes and the password is no longer working. The Guardians start to panic and the Doctor realises they’re now prisoners of these dimensions and will die like everyone else if they don’t get off the ship. The Doctor finds another jewel like the one in the necklace, and tells Guardians to take it down to the far end of the spaceship while he prepares to fire the projectile guns. With any luck, it’ll blow a hole in the hull.

The two Guardians head for the front of the ship and come across another room filled with treasures taken from other worlds. The White Guardian points out that if the Black Guardian had given just part of this to the Caliph instead of the gold, there would have been no trouble. Unfortunately these are the private quarters of the Djinni and the Black Guardian had never been to this part of the ship before. They throw down the tracker and then run for their lives. The Doctor powers up the guns and Amy asks if she can press the button. She does so and the projectile fires straight up into the air, then turns and heads back down towards them. There’s a massive explosion and the ship is filled with smoke and debris. The Guardians pick themselves up of the floor and discover the front of the ship is now exposed to the desert, exactly as the Doctor had intended.

The Doctor calls to everyone and warns them to get as far away as they can. Once they reach a safe distance, he tells Amy to convert the gold shard back into the fifth segment. Meanwhile, Omar and Nisrin have ignored the Doctor’s warning and are continuing to collect as much of the treasure as they think they can carry. Nisrin points out that what they’re doing is stealing, but Omar says everyone here is doing the same thing and it could be said that this cave has forty thieves. Nisrin decides it’s about time she saved her own life for a change and tells Omar that if he loves her, he should follow her now. Omar eventually does what she says, shocked that a slave could order him what to do.

Amy places the fifth segment in her satchel, then the Doctor leads her back into the ship to look for something. The Guardians had earlier told him they followed him to Earth after finding his TARDIS drifting nearby in space and he’s sure the Djinni would have considered it to be eminently collectible. His theory is proved correct and eventually they find the TARDIS stored in one of the treasure chambers. They clear the junk away from the door and let themselves in, then the Doctor dematerialises his ship just as the countdown reaches single figures…

Omar and the others are racing across the sand dunes when the two Guardians suddenly stop as they sense the fifth segment somewhere nearby. Together, they turn and curse the Doctor, just as the buried spaceship explodes. Omar cheers in delight as silver coins start raining down on them from above. Nisrin rebukes him for learning nothing as the falling treasures are enough to kill anyone they strike. The group takes cover and Omar finally accepts that Nisrin is indeed his better half. He tells her she no longer needs to call him her master and from now on she can call him Prince Omar.

Inside the TARDIS, the Doctor and Amy congratulate each other. The Doctor sets about looking for the last segment, confident that the two Guardians will remain stuck on Earth until they can complete and repair the Key to Time.