Sarah, investigating the TARDIS's wardrobe, has found a dress previously worn by the second Doctor's companion Victoria. She is showing it off to the Doctor when the TARDIS is buffeted by an external force and a hideous jackal-like face appears in the console room, terrifying Sarah.

The TARDIS arrives on Earth, but the alien contact has pushed the ship off course slightly, and instead of arriving at UNIT HQ in the present day, the Doctor and Sarah find themselves in an old priory in the year 1911. The priory stands on the same site that UNIT HQ will occupy in the future.

The priory belongs to Professor Marcus Scarman, an archaeologist and Egyptologist. Scarman is in Egypt and a mysterious Egyptian, Ibrahim Namin, is looking after his affairs until he returns. Laurence Scarman, Marcus's brother, grows suspicious when he finds that he is unable to contact him. He confronts Namin in the priory but can get no information from him. When Laurence has left, Namin activates his servants - walking mummies - and prepares to speak to his true master via an upright sarcophagus. The sarcophagus is actually the portal to a space/time tunnel through which Sutekh the Destroyer, an Osiran worshipped as a god by the ancient Egyptians, plans again to walk the Earth. Sutekh took over the mind and body of Marcus Scarman when the latter excavated the pyramid in which he was trapped, and he sends this walking cadaver back to Namin via the space/time tunnel. Namin is killed and Sutekh, through Scarman, sets about making arrangements to free himself from the pyramid where his brother Horus trapped him aeons ago.

Sutekh seals off the priory from the outside world and instructs his servicer robot servants (the mummies) to construct a rocket which will destroy a pyramid on Mars which is sending a signal holding him trapped on Earth. The Doctor tries to stop the rocket by detonating a box of explosive in it, but Sutekh prevents the explosion by force of will and the Doctor has no choice but to travel to Sutekh's pyramid via the space/time tunnel and distract the Osiran so that the rocket will be destroyed. This places the Doctor under Sutekh's control and he is returned to the priory in order that he may transport Scarman to Mars. There, Scarman can ensure that the bonds which hold Sutekh are finally broken.

Scarman, with Sutekh's help, navigates the puzzles which guard the way into the pyramid on Mars and switches off the power. The Doctor, now free of Sutekh's influence, realises that there is a time delay between Mars and Earth. He rushes back to Earth and connects equipment from the TARDIS to the space/time tunnel, extending its end point into the far future. Sutekh, travelling down the tunnel, is unable to reach the end in his lifetime, and dies. The energy from the tunnel sets fire to the priory which burns down as the Doctor and Sarah leave.

_{Synopsis from Doctor Who: The Fourth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}