An army of mercenaries is massing outside the Egyptian city of Thebes, under the leadership of the brutal Yanis; but Yanis’ ally, a temple priest named Horemshep, has found something that could change their plans completely. To Yanis, it appears to be nothing more than a simple box -- until he opens it. Back in Thebes, in the Pharaoh’s palace, the guard commander Antranak finds the junior priest Fayum staring at the night sky in concern, seeing terrible omens of things to come...

The Doctor gives Peri a tour of the TARDIS, but it proves more difficult than he’d thought, as the TARDIS has been moving rooms around again and there’s a small lake where the main library used to be. The ship suddenly lurches for no apparent reason, and when the Doctor returns to the console room to investigate, he finds that the TARDIS has changed course -- apparently on his orders. Curious, he sets the co-ordinates to go back in time a few days so he can find out what happened. The TARDIS materialises in an abandoned house in Egypt, circa 1400 BC, and as the Doctor and Peri explore they hear a girl screaming for help. Her chariot is running out of control, and she’s being pursued by a man who clearly doesn’t have her best interests at heart. The Doctor and Peri string a rope between two houses, let the girl past and then pull the rope tight, knocking the man from his chariot. While Peri keeps an eye on the man, smashing jars over his head if he looks as though he’s starting to stir, the Doctor takes the man’s chariot and follows the girl. Her horses are too frightened to stop running, but the Doctor talks her into leaping across to his own chariot, thus saving her life. Shaken by her experience, and surprised that the Doctor doesn’t know who she is, the girl tells him to take her to Thebes, where all of his questions will be answered.

The Doctor returns for Peri, but there isn’t room on the chariot for all four of them and more mercenaries are approaching; thus, they must leave the girl’s attacker where he lies. Yanis is forced to call off the pursuit as his enemies near Thebes, knowing that the city archers would cut down his men as they approached; but he’s seen the face of the fair-haired man who robbed him of his prize, and he vows revenge. First, however, he must punish Kishik, the man who let the girl escape. Kishik pleads for mercy, pointing out that the girl had a larger escort than anticipated; he’d been ordered not to attack until Yanis arrived, but he feared that she would escape if he delayed any longer. Yanis must concede that all of the girl’s escort lie dead and that the man who saved her rode a chariot like a true warrior; however, the girl has escaped in any case and now she will alert the forces at Thebes to their presence. Yanis still has need of Kishik, however, so rather than gut him and leave his body for the dogs, he takes Kishik to the shrine in the back of his tent, where Kishik sees something terrifying. From now on Kishik will be of far more use to Yanis...

The Doctor and Peri are surprised when the people of Thebes immediately prostrate themselves before the girl upon their arrival, and even more surprised when the palace guard arrive and greet the girl, Erimemushinteperem, as Pharaoh. Erimem recognises that she owes the Doctor her life and insists that he and Peri be treated as her personal friends, and once they reach the palace she orders that a banquet be held to honour the strangers who saved her life. Antranak has been a palace commander for too long to trust to good luck, and is suspicious of the strangers’ sudden arrival; thus, while he publicly supports Erimem’s decision to trust them, he privately orders his guards to keep a close eye on them. The Doctor and Peri prepare for the banquet, but while Peri appreciates being pampered and tended by servants, she realises that the Doctor seems ill at ease. When pressed, he admits that he’s been trying to remember the names of Egypt’s female Pharaohs, and can’t remember one named Erimem. Before her official coronation, something will happen to prevent her from taking the throne...

The strangers’ arrival is the talk of court, and Horemshep, High Priest of Horus, seizes the opportunity to bait Antranak; why does Erimem honour these new arrivals with a banquet when she has never done the same for the loyal Antranak? Fayum diplomatically intervenes, realising that, although Antranak knows Horemshep is jealous of his influence and is trying to bait him, he’s close to losing his temper in any case. Officially the strangers must be treated with all courtesy, as Pharaoh is divine and therefore infallible; but there’s already a “debate” in the council of priests over Erimem’s right to take the throne. It hasn’t developed into a split, not yet...

As the Doctor passes out, the assassin takes poison himself, claiming aloud that he serves the true Pharaoh. With Peri’s support, the overwhelmed Erimem pulls herself together and orders Antranak to torture the truth out of the assassin. She and Peri accompany the unconscious Doctor back to Erimem’s chambers, while Fayum frets over the assassin’s words -- his claim to serve “the true Pharaoh” will be feeding the rumour mill within hours. Even now those at court are already discussing it. The last year has seen great unrest, and this incident will only make matters worse and may strengthen the position of those who oppose Erimem’s coronation. Fayum will do what he can to quash those rumours, but first he must lend his medical training to tend to the dying Doctor, while Antranak takes the assassin to the “house of death” to be tortured. Outside the city, Kishik reports the plan’s failure to Yanis, only to find that Yanis already knows that Erimem is alive -- and that her new champion is as good as dead...

Fayum tends to the Doctor, and like most people is surprised to hear two hearts beating in his chest. His pulse and breathing are weak, and there’s no cure for this venom; Peri helplessly realises that toxins and penicillin remain undiscovered in this era. As Fayum leaves to attend an emergency council meeting, Antranak returns to announce that the assassin remains silent. All he knows is that the assassin was spotted approaching the banquet hall from the palace of concubines, and was therefore assumed to be a eunuch. Only eunuchs and Pharaoh may enter the palace of concubines, and for this reason Antranak has been unable to question the concubines himself. Peri therefore offers to do so, but this would be an unacceptable breach of custom, and Erimem therefore insists upon accompanying her. Antranak is unwilling to let Erimem place herself at risk, but her word is law, and while she appreciates his concern she will not be moved on this matter.

As Antranak reluctantly prepares an escort of his most trusted guards, Fayum returns from the council meeting with bad news. Antranak brushes irritably at a fly buzzing about their heads as Fayum reveals that Erimem’s enemies have seized upon the assassin’s claims to insist upon a delay to the coronation. Her supporters have been forced to concede, knowing that as long as the rumours of a “true heir” fly about the city Erimem will be unable to take the throne with the full support of the people. For once, Antranak must agree with Fayum, but he’s tired and he blames himself for this appalling breach in security and for caving in to Erimem’s demand to visit the palace of concubines. As he prepares to return to his Pharaoh, he loses his temper and strikes down the buzzing fly with his sword... but it has already done its work. Yanis has overheard the entire conversation, and he now knows what Erimem intends to do. As Kishik organises the troops to surround Thebes, Yanis uses a miraculous device provided by his new ally to send his “smallest soldiers” on a new mission. When the mercenaries attack, the Egyptians will be in no state to defend Thebes, for they’ll be mourning Erimem’s death...

Antranak and his escort lead Erimem and Peri to the palace of concubines, which is closed up and silent; the concubines are mourning Varela, who took her own life with scorpion venom a few hours before. Despite the sad circumstances, Peri and Antranak agree that they can’t wait any longer to learn the truth; Antranak knows of the existence of a second, secret exit which any of the women could use to slip away unnoticed. A mute eunuch arrives to let Erimem and Peri in; Erimem must enter first, and as Peri prepares to follow, Antranak privately warns her that she will answer to him personally if any harm comes to Erimem. Inside the concubines’ palace, the sound of wailing disturbs Erimem and Peri, who decide to investigate the second exit rather than disturb the mourning women any further. As the eunuch leads them to the exit, Peri realises that she and Erimem are now on their own; no soldier may enter this palace on pain of death or castration. Meanwhile, Fayum tends to the comatose Doctor, but fears that he will be unable to find a cure as Erimem has ordered. But then something surprising happens...

The secret passage from the palace of concubines is over 500 years old, and is decorated with thick tapestries woven by generations of women from the palace above, thick with dust disturbed only by the assassin’s footprints. As they proceed, Erimem asks Peri about the Doctor, and is surprised to learn that Peri is neither the Doctor’s wife, concubine, nor slave. She’s full of questions about Peri’s home, and is fascinated to learn that every four years the common people are allowed to choose who will rule them. Peri in turn asks Erimem about her own claim to the throne, and learns that Erimem has recently been beset by tragedies; her three half-brothers died of various ailments and accidents, and her father, apparently driven to extremes by grief, was killed by mercenaries when he failed to take the proper precautions while riding outside the city. Erimem is the only living heir to the throne, and is considered divine by everyone but herself; even her own mother won’t look her in the eye. But a god would not have needed the Doctor to save her life, and would not feel as afraid as Erimem does now.

The passageway widens into a chapel where the concubines worship their gods, but as Peri steps forward to examine the tapestries and the altar, she and Erimem hear a scuttling sound -- and thousands of scorpions rush into the chamber, blocking their escape. Peri and Erimem climb up onto the altar, but the scorpions, acting with uncharacteristic co-operation, climb atop one another to create a living pyramid; soon they’ll reach the top of the altar. Thinking quickly, Peri and Erimem tear down one of the tapestries, throw it over the scorpions and rush across its surface to the exit. They manage to outrun the scorpions, and see light at the end of the tunnel; it emerges at the base of the western hills, and morning is breaking. Erimem can see the market opening in the city. A short walk will take them back to the palace.

Once again, Yanis knows of the girls’ escape, and it hasn’t put him in a good mood. Enraged, he orders Kishik to have his men surround the city. Meanwhile, Horemshep arrives in the house of death and murders the guard watching the assassin; but the assassin is in no condition to escape, so Horemshep kills him to prevent him from talking.

Even as Antranak speaks, however, a very healthy Doctor and a very bewildered Fayum are taking a chariot to the mercenary camp. Fayum can’t understand how the Doctor survived, let alone sat up and asked for breakfast, but he’s promised to take care of the Doctor and won’t let him go out alone. They abandon the chariot at a nearby oasis and proceed stealthily on foot; as they do so, the Doctor notes that the mercenaries have Thebes surrounded to the south and the west. The Nile blocks the east, and as there is a second Egyptian garrison to the north at Giza, the mercenaries’ leader has wisely chosen not to sandwich his men between two enemy forces. Since most of the mercenaries are in position to attack the city, the main camp is almost deserted. Fayum kills the one guard who spots them before he can sound the alarm, upsetting the Doctor; he’s here to prevent bloodshed, not cause it.

The largest tent in the camp is presumably the leader’s, and it’s full of jewels and gold, the spoils of previous campaigns. The Doctor is more interested in the second room, where corpses lie piled behind what appears at first glance to be a metal shrine. At second glance, it’s far more than that. Yanis catches the Doctor and Fayum trespassing, and soon realises that the fair-haired stranger who shows no fear must be Erimem’s new champion. The Doctor warns him that his true enemy is the creature he found in the “altar”; the Doctor realised something was amiss while unconscious, when his wandering mind detected a strong telepathic presence in the vicinity. The “altar” is a stasis box from a prison ship, and while Yanis might think that the thing inside gives him the power to rule worlds, it will eventually rule Yanis himself. Yanis refuses to listen to the Doctor’s warnings, but before he can kill the intruders the Doctor removes a device from the stasis box -- a telepathic inhibitor which, when activated, sends Yanis and his fellow mercenaries into convulsions. The Doctor and Fayum steal a chariot and ride back to the oasis, where their own, faster, chariot is waiting, with fully rested horses ready to take them back to Thebes with their warning.

As Erimem and Peri share their concern for the missing Doctor, Antranak arrives with appalling news. The mercenaries have sent an ultimatum threatening to slaughter every living Egyptian unless the forces of Thebes surrender immediately, but the council of priests is still hesitant to commit its troops, knowing that the people will never agree to depose Erimem if she emerges from the battle victorious. Furthermore, the priests dare to summon Erimem to an emergency council meeting, as if she were no more than a common servant. Despite this outrage, Erimem agrees to attend the meeting, and Peri agrees to accompany her, knowing that Erimem needs a friend. Meanwhile, Yanis orders Kishik to hold position outside the city. There are many ways to win a battle, and they’ll give their ally inside the city a chance to try his way first...

Erimem arrives at the council meeting, and unsurprisingly, Horemshep is the one who rises to confront her. There is already much controversy over a female’s right to rule as Pharaoh, and the assassin’s claim thus had to be investigated fully -- and Horemshep claims to have found evidence of another heir to the throne. The late Varela, who so recently died in the palace of concubines, took a rare venom which is only known to a few apothecaries -- and to certain of the palace guards, including their commander. Varela knew of the true heir to the throne, and Horemshep accuses Antranak of poisoning her to keep her silent. Erimem can’t quite believe this, but she knows that Antranak is sometimes too protective of her; though she said nothing at the time, she heard him threaten Peri outside the palace of concubines. She doesn’t want to believe him capable of murder, but how far would he go if he thought she was in danger?

It is at this moment that the Doctor arrives, carrying Pharaoh’s cartouche of protection, and ready and willing to act as Erimem and Antranak’s counsel. Peri, relieved to see him alive, brings him up to date, and the Doctor presses Horemshep for proof of his claims. Horemshep reveals that poison was found in Antranak’s chambers, and he has already shown that he knows of the second, secret entrance to the palace of concubines. The Doctor finds it odd that Antranak would keep incriminating evidence in his own rooms, but passes over this for the moment while Horemshep reveals the motive for the murder; Varela was keeping letters from the late Pharaoh which reveal that she had borne him a son in secret. This son, Erimem’s half-brother, is the true heir to the throne -- and Horemshep triumphantly reveals that he is Fayum, who was taken from his mother at birth and raised by a temple priest, ignorant of his true parentage. Fayum, shocked, protests that this cannot be true, but Horemshep grips him and shakes him, demanding that he acknowledge his destiny. The Doctor watches carefully as Fayum, suddenly dazed and disoriented, starts hearing voices in his head -- and steps forward to accept his role as the Pharaoh of Egypt.

His suspicions confirmed, the Doctor steps in to put an end to this nonsense. Fayum tries to move him out of the way, but finds that he has no influence over the Doctor, who is more than a match for the real enemy he now knows he’s facing. The Doctor loudly questions the veracity of Horemshep’s “evidence”, which so conveniently happened to be lying around at the right moment. The priest claims that divine intervention led him to the truth, and the Doctor thus challenges him to let the gods choose the true Pharaoh. Privately, he tells Peri and Erimem to trust him, and as he reverses the settings on the telepathic inhibitor from Yanis’ tent, the girls taunt Horemshep into calling on the sun god to show his favour to Fayum. Horemshep does so -- and the sun goes out. The Doctor, struggling to concentrate, tells Erimem to command the sun to return; astounded by what she’s seeing, she nevertheless plays her part regally, and the sun returns to the skies. As the Doctor recovers from the strain, he explains to Peri that he used the telepathic inhibitor to contact the TARDIS and reprogramme it by remote control; this was the cause of the jolt which they experienced earlier, which led them to Egypt in the first place. The Doctor reached into the past and used the TARDIS to block out the sun.

Horemshep protests that this was a trick, but he’s clearly lost, and the council of priests is now far more willing to listen when the Doctor accuses him of killing Varela and forging the letters in order to replace Erimem with a Pharaoh whom Horemshep could easily manipulate. He also accuses Horemshep of striking a deal with Yanis, to use the mercenary army as a visible threat to unsettle the city while he stirred up trouble inside. Why else would the army stand by without attacking? The game is lost, but Fayum and Horemshep now have a new agenda -- they must acquire the Doctor’s intelligence for themselves. Fayum takes Peri hostage, threatening to slit her throat unless he and Horemshep are granted free passage from the city. The guards stand by as the conspirators flee with their hostage, but the Doctor advises Erimem to mobilise every soldier and chariot she can; war is about to be declared, and unless they act quickly Peri will be trapped on the wrong side of the lines.

Peri wisely leaps from the chariot as Fayum and Horemshep flee, knowing that a few bruises and scratches are preferable to what will happen when they no longer require a hostage. Once she’s reunited with her friends, the Doctor advises Erimem and Antranak to march their army to Giza to meet up with the garrison there. Antranak refuses to abandon Thebes to the mercenaries, but the Doctor insists that this is the only way; the mercenaries outnumber the Egyptians here, and Antranak’s men will be slaughtered if they engage in battle now. Peri urges Antranak to trust the Doctor, but only when Erimem gives him a direct order does Antranak reluctantly prepare his men for the long march. The Doctor sends Peri with Antranak while he stays with Erimem, and once the army is assembled, they set off.

It’s time for the Doctor to reveal the true nature of the enemy to Erimem, and it’s an enemy like none she has ever encountered -- a being of pure thought, and an escaped prisoner to boot. A prison ship must have crashed somewhere on Earth, freeing the prisoner from its stasis box and allowing it to begin the process of possession, passing from one host to another like an infection and spreading a vast web of telepathic connections as it goes. Fayum was infected when Horemshep touched him, and Peri must have been infected when she was taken hostage. Those with a strong will can resist the creature’s control for a while, but eventually the host’s personality and memories will be subsumed into the gestalt. This is beyond anything Erimem has ever heard, and she is now certain that the Doctor is more than a mere mortal -- yet she does not believe in the gods. He promises to tell her of his travels one day, but first they must ensure that Peri does not betray their plans to the mercenaries...

Peri asks Antranak apparently innocent questions about his plans and the size of the garrison at Giza, and Yanis, Fayum and Horemshep hear every word that passes between them. Soon they will know all that Peri knows, but her mind is still too strong for them to read it entirely; all they can tell is that she and the Doctor are space travellers. With the Doctor on their side, the creature will be able to travel through space and conquer other worlds once again. Night falls, and as the Egyptian army sets up camp, Yanis must do the same; he’d prefer to march all night and attack the unprepared Egyptians at dawn, but the entity needs living hosts and knows that too many of the mercenaries will die without rest. Yanis reluctantly gives the order to rest, but the entity knows that he’s too violent and strong-willed; if he continues to resist, the entity will cut its losses and kill him. Meanwhile, as the Egyptians settle in for the night, the Doctor drugs Peri’s food so she won’t hear him discussing the battle to come with Erimem and Antranak.

Morning dawns bright and cheerful, as does Erimem, who spent a fascinated night listening to the Doctor’s tales of Daleks and Cybermen. As they near the tombs and the Sphinx, however, she admits that she understands the implications of the fact that the well-travelled Doctor has never heard of her. Today she may die, and even if she lives she will never be Pharaoh; yet this doesn’t trouble her, for she’s always felt that her destiny lay elsewhere. Antranak and Peri report that the mercenaries are preparing to attack, but there is still time for the Doctor to reach the well behind the Sphinx; it’s been abandoned by locals who believe the sacred cats have drunk it dry, but the Doctor reveals that it’s no well at all, but a sink shaft. The Sphinx was built by refugees from Atlantis, and hidden beneath its paws is a hall of records containing information about a weapon which the Doctor can use to defeat the entity. Erimem and Antranak prepare to hold off the mercenary army, while Peri, unsurprisingly, chooses to accompany the Doctor.

What Peri knows, the entity knows, and the Doctor must be prevented from seizing the Atlantean weapon. Yanis thus orders his weary troops to attack immediately. The Egyptian troops try to avoid hand-to-hand combat for fear of becoming infected, but Erimem refuses to stand by while other men die at her command, and Antranak watches proudly as she dons battle armour to fight alongside her people. The battle begins with the Egyptian archers cutting down enemy chariots as they approach; however, they can’t hit every chariot and soon the enemy is too close for the archers’ weapons to be of any further use. However, a stray arrow takes down Kishik, and the enraged Yanis orders his men to kill every Egyptian they can, despite the entity’s need for more living hosts. The entity manages to force Yanis back under control, and sends him to the sink shaft with Fayum to stop the Doctor. Erimem and Antranak spot him, and Erimem orders Antranak to send in the infantry while she stops Fayum and Yanis -- but Horemshep has slipped behind enemy lines and he strikes down Antranak as they speak. Before Erimem can attack in turn, Yanis captures her; but he doesn’t kill her, for she will make an excellent hostage when they confront the Doctor.

The Doctor and Peri descend the shaft and make their way along millennia-old tunnels, taking care not to disturb the wild cats or the ancient tunnel supports. They eventually reach what appears to be a blank wall, which the Doctor claims is really the door to the Atlantean hall of records. It hasn’t been opened for 8500 years, but Horemshep and his allies now arrive, with Erimem as their hostage, and demand that the Doctor open it and give the weapon to them. When he promptly agrees to do so, Horemshep becomes suspicious, fearing that the Doctor has led them into a trap. Before they take the weapon they must take the Doctor’s mind, to be safe. But no one mind can defeat the Doctor, and the entity thus pulls every fragment of its gestalt out of those it has infected, putting all of itself into one host in order to overpower the Doctor. Yanis and Fayum faint as the Doctor turns on Horemshep -- but the entity has anticipated this, and has possessed Peri instead, knowing that the Doctor will never harm a friend...

The Doctor, desperate, offers the entity free passage away from Earth to a world inhabited by non-sentient life, but it rejects the offer; it will not make its home in blank minds. The Doctor has no choice but to activate the telepathic inhibitor, ignoring the entity’s attempt to convince him that this is harming Peri. Horemshep makes a run for it as the machine powers up to full strength, but the entity is now too strong to succumb to it; however, its taunting gives the Doctor an idea. The entity has the knowledge of thousands to draw on, but right now it’s inside a single human mind, which isn’t designed for more than one set of memories. The Doctor and Erimem call out to all of the minds in the gestalt, asking them personal questions and changing tack before the entity can respond. Peri’s mind can’t handle the conflict between so many separate memories, and she collapses, overwhelmed. The entity manages to pass a small bit of itself into a passing cat, but the Doctor catches it and ties the inhibitor around its neck, trapping the entity until he can hand it over to the proper authorities.

Yanis recovers and tries to kill the Doctor for cheating him of the great power at his command, but Fayum has recovered as well and engages Yanis in battle. Much to everyone’s surprise, he wins. Yanis stumbles into a tunnel support as he falls back, bringing the ancient roof down on his head, but the Doctor, Peri, Erimem and Fayum get to safety before the tunnel collapses completely. Outside, they find that the battle is over; with the entity gone from their minds, the mercenaries fled in terror when the earth tremors from the collapsing tunnel cracked the face of the Sphinx. Antranak has survived, and he’s captured Horemshep, who tries to claim that his treachery was only the result of demonic possession. Nobody believes him, and he’s dragged off in chains while Fayum is forgiven, having proven himself by fighting and defeating Yanis. As the Egyptians prepare to return to Thebes, Erimem asks the Doctor if he made up the story of the “Atlantean weapon” in order to trick the entity into committing itself to one body... but the Doctor doesn’t answer.