The TARDIS materialises in Victorian times in the Egyptian wing of the British Museum, and the Doctor discovers that it has been drawn off-course by an Osiran lodestone sarcophagus. While he and his friends are exploring, however, Nyssa is kidnapped and carried out of the museum by unidentified attackers. As the Doctor and Tegan search vainly for her in the gaslit streets outside, they are contacted by a butler named Atkins -- who seems to know them already, and who gives them an invitation to a mummy-unwrapping party at Lord Kenilworth’s home the next afternoon. Atkins seems to believe that the Doctor and Tegan are staying at the Savoy, and when they investigate they find that rooms have already booked in their names and the hotel staff seem to know them already. One waiter in the dining room is particularly rude to Tegan, due to the displeasure she expressed the previous night when he served her cutlets. Puzzled, the Doctor and Tegan attend the party the next afternoon, where Lord Kenilworth thanks the Doctor for his help on the expedition. The mummy is then unwrapped -- to reveal Nyssa, who has been in a deep metabolic coma for over four thousand years. The Doctor tries to wake her, but realises that it will take a hundred years for her metabolism to get back up to speed. Lord Kenilworth promises that he and his family will watch over her in the meantime, and takes half of the Doctor’s invitation to the party; in a hundred years’ time, his descendent will deliver the mummy into the care of the man who arrives with the other half.

The Doctor and Tegan return to the Museum, but after they leave, Kenilworth realises that he’s forgotten to ask whether it’s important for the sarcophagus lid -- now on loan to the Museum -- to be replaced. He thus sends Atkins after them to inquire, but just as Atkins catches up they are attacked by Nyssa’s assailants -- Egyptians led by Sadan Rassul, who recognises Atkins even though the butler has never seen him before. Rassul has already used the lodestone to send Nyssa back in Time, and he now seals the Doctor, Tegan and Atkins in what he assumes to be another sarcophagus and drops it into the Thames to prevent them from interfering further. Fortunately, the “sarcophagus” is actually the TARDIS, and the Doctor is thus able to follow Nyssa’s trail through Time back to Ancient Egypt. Atkins has no choice but to accept that he’s travelled in Time, and he accompanies the Doctor and Tegan as they head for the nearby pyramids to investigate further. They are too late to save Nyssa, who has already been placed in a coma by the high priest of Nephthys -- Sadan Rassul. The burial ritual involves the ceremonial breaking of a canopic jar, and a handmaiden who had befriended Nyssa as she was prepared for sacrifice explains to the Doctor that the jar used to contain the essence of the goddess; however, it cracked some time ago, and Nyssa has thus been chosen as the new host.

The Doctor takes the TARDIS to Victorian-era Egypt to join Lord Kenilworth’s original expedition, and although Kenilworth is surprised to see his butler Atkins in Egypt, Atkins convinces him to listen to the Doctor. Kenilworth, impressed by the Doctor’s knowledge of enthusiasm, and when the Doctor offers to lead Kenilworth directly to the lost pyramid he’s seeking, Kenilworth agrees to let him join the expedition. When they arrive, the Doctor finds it disturbingly easy to open the pyramid, but inside, the expedition members are sealed inside the entrance tunnel and confronted by two Shabti, automaton guardians carved to resemble a beautiful young woman. The ceiling of the tunnel is illuminated with a pattern of lights, and the intruders are ordered to select the missing point. Nicholas Simons identifies the pattern of lights as the position of the Great Pyramids along the Nile, but the Doctor recognises that they are also stars in the constellation of Orion. The missing point represents the location of the Sphinx, or of the planet Phaester Osiris, and when the Doctor touches the space where it should be, the doors of the tomb open up -- releasing a blast of psychic energy which kills Simons before he can get to shelter with the others.

Over the course of the next few days, most of the artefacts in the tomb are excavated, although the Doctor is pained by the archaeologists’ casual belief that it’s better to move the artefacts to Britain than leave them in context in the sands of Egypt. While spending time in the tomb she knows to be her friend’s, Tegan finds a secret panel which opens up to reveal another tomb with a second mummy inside. The Doctor becomes concerned when he sees that the cartouche on the tunnel wall identifies this as the resting place of Nephthys, and he secretly alters Kenilworth’s notes to ensure that the second mummy will not be discovered. As the expedition wraps up and prepares to set off, however, Rassul arrives, reanimates Simons’ corpse and activates a number of Osiran service robots disguised as mummies; although he intends to let Kenilworth take Nyssa’s mummy back to England, some of the artefacts he’s taken must be returned to the tomb to ensure that its psionic potential does not degrade. With the Doctor’s help, the expedition members fight off the attack with few casualties, and retreat to Cairo with a number of artefacts still in their possession.

The Doctor, Tegan and Atkins accompany Kenilworth back to London, but since Atkins is unable to return to Kenilworth’s home while his younger self is there, he waits in the TARDIS while the Doctor and Tegan help prepare for the mummy-unwrapping party. When Kenilworth arrives at home, he’s surprised to find Atkins waiting there, claiming that he never left. The Doctor books rooms for himself and Tegan at the Savoy, where Tegan tries to defy destiny by ordering ham and subsequently loses her temper with the waiter when he mishears her order and serves her lamb cutlets. The Doctor tells Kenilworth to deliver an invitation to him outside the British Museum at midnight, and returns to the TARDIS with Tegan, having ensured that events will proceed for him as they did before.

The Doctor, Tegan and the elder Atkins then travel forward in time to 1996, to wait for Nyssa to awaken. The Kenilworth family home is now owned by a geneticist named Aubrey Prior, and has undergone extensive renovation following a fire which gutted its interior. When the Doctor supplies Prior with his half of the invitation, Prior invites him and his friends to attend his daughter Vanessa’s 21st birthday party. There, they meet her fiancé, a young architect named James Norris, and recognise Vanessa’s ring as one of the artefacts from the tomb.

Prior has developed an interest in Egyptology due to his legacy, and he shows the Doctor and Tegan that he’s surrounded Nyssa’s mummy with plasterboard recreations of the original tomb walls... including the cartouche of Nephthys which the Doctor erased from the expedition’s notes. The Doctor is beginning to fear that he’s made a terrible mistake in reviving Nyssa. He explains to Tegan and Atkins that much of Egyptian culture was shaped by the Osirans, aliens with godlike powers whose terrible war with the renegades Sutekh and Nephthys finally ended on Earth. The Osiran leader, Horus, cruelly imprisoned the renegades with the means of their escape just barely beyond their grasp. The Doctor has already fought Sutekh and won, but according to legend, Nephthys was even more cunning, devious and dangerous. The Doctor is unsure just how Nephthys was imprisoned, but he believes that Nyssa is somehow involved -- and it’s very possible that by breaking her coma, the Doctor may have inadvertently provided Nephthys with the means by which to free herself.

Over the past century, the artefacts from the tomb have passed into the hands of private collectors. When one such collector is murdered and the artefact stolen from his collection, the Doctor realises that someone is gathering together the psychic foci required to restore Nephthys to life. There are only two remaining in private hands -- Vanessa’s ring, and a jade cobra belonging to James Norris. The Doctor sends Tegan to warn Norris of the danger, while he and Atkins visit contemporary Egypt to find out how Prior learned about the “missing” cartouche. There, they learn that Prior led an expedition to the pyramid some years ago, and removed the second mummy and tomb’s original walls, claiming that they were to be reassembled in the British Museum. The Doctor explains to Atkins that the tomb is useless where it sits now; the Sphinx and the Great Pyramids are part of a psionic relay system which the Osirans used on Earth, but over the past five thousand years, Earth’s position relative to their constellation has shifted, and the relay system should no longer be able to draw power from their homeworld.

Tegan arrives at Norris’ country home just as an entranced Vanessa tries to steal the jade cobra. Norris tries to restore Vanessa’s sense of self, but she still feels odd compulsions pulling at her mind, and loses consciousness. An Osiran service robot commanded by Rassul then breaks into the house, knocks out Tegan and Norris, and takes away both Vanessa and the cobra. When Norris recovers, he explains to Tegan that he originally brought the cobra to Prior for identification, and subsequently fell in love with Vanessa when Prior hired him to redesign his house after the fire. But over the past few days she’s been behaving strangely, and while trying to get a passport for their honeymoon, Norris found that there were no records of her birth on file, and no legal papers indicating who her mother was. The Doctor and Atkins then return and reveal that, having researched Egyptian mythology in the TARDIS library, they now believe they know how Nephthys was imprisoned. Her mind was split in two, her intellect and reasoning stored in a canopic jar while her instinct and intution were stored in an innocent, unsullied human mind. Rassul was given eternal life in order to guard her tomb, but when the canopic jar was cracked during a failed tomb robbery, centuries after her imprisonment, some of Nephthys’ influence was released, and Rassul fell under her influence. He thus selected Nyssa to host the other half of Nephthys’ mind -- choosing a prison which Nephthys knew she would be able to escape.

The Doctor departs to collect the equipment he needs for the final battle, and travels to 1926 to ask a favour of an old friend. Tegan, Atkins and Norris confront Prior, only to find Rassul waiting. He used to be Horus’ high priest, until Horus selected Rassul’s daughter to host Nephthys’ instinctive half; Nephthys thus used his guilt to bind him to her service. When Prior returned from Egypt with the second mummy, he ran a CAT scan on the body, and in so doing released enough of Nephthys’ psychic potential for Rassul to enslave him. Under Rassul’s influence, Prior took tissue samples from the second mummy, and created a clone -- Vanessa. He also set his own house on fire so he would have an excuse to rebuild it into the shape of a pyramid. Over the years, as Earth’s position has shifted, the focal point of the Osiran relay has moved from the Sphinx to this house. Vanessa’s mind has now been entirely subsumed beneath Nephthys’ instinctive half, and when Nyssa awakens Nephthys’ mind will be fully restored -- as, Rassul hopes, will be the original host, his daughter.

Rassul shoots and kills Norris, and leads the subdued Tegan and Atkins to the cellar, where the Doctor is waiting for them. Vanessa kills Prior, and waits for Nyssa to awaken -- but when Nyssa rises from her resting place, the wrappings fall away to reveal a ninety-year-old woman. The Doctor announces that she’s been in a waking trance since 1926, ageing while remaining dormant, and that over time Nephthys’ reasoning side has dispersed. Unable to locate any trace of herself in the woman before her, Vanessa/Nephthys uses the sarcophagus lodestone to travel back to 1926, believing that she will become complete when Nyssa wakes. However, when she arrives in 1926 she finds Nyssa still in a deep coma, from which she will not awaken for 70 years. Acting entirely on instinct, with no intellect or reason to hold her in check, Nephthys repeatedly travels back and forth between 1926 and 1996, between the sleeping Nyssa and the aged woman -- but although each journey takes moments she still ages seventy years, and eventually she crumbles to dust, having aged beyond the lifespan of even an Osiran. Rassul and the mummies fall to dust at the same time. The Doctor then reveals that the aged woman is in fact their old friend Lady Ann Cranleigh, Nyssa’s exact double, whom they met in 1926. The real Nyssa is in the TARDIS, and the Doctor uses a canopic jar which he stole from the British Museum and modified to entrap Nephthys’ intellectual side when Nyssa awakens. The Doctor buries the jar in the sands of Egypt, near the original pyramid, and returns Atkins to his home with a newfound appreciation for adventure and life.