An antimatter creature has crossed into normal space via a phenomenon known as the Arc of Infinity but needs to bond physically with a Time Lord in order to remain stable. A traitor on Gallifrey has chosen the Doctor as the victim.
Tegan falls once more under the influence of the Mara and directs the Tardis to the planet Manussa. There the Federator’s son Lon and his mother Tanha are preparing for a ceremony to celebrate the banishment of the Mara five hundred years earlier. It’s up to the Doctor to defeat the evil Mara.
The Black Guardian recruits a young man named Turlough to assassinate the Doctor. Although outwardly an ordinary pupil at a boys’ private boarding school, Turlough is in fact an alien who believes that the Guardian will return him home if he succeeds.
The TARDIS attaches itself to a space liner after Turlough, still under the Black Guardian’s influence, damages its controls. The Doctor and Nyssa meet two space pirates, Kari and Olvir, who have come on board the liner in search of plunder, while Tegan and Turlough get lost in the infrastructure.
The White Guardian warns of impending danger and directs the TARDIS to what appears to be an Edwardian sailing yacht, the SS Shadow, but is actually one of a number of spaceships taking part in a race through the solar system, the prize being Enlightenment. The yacht’s Captain Striker and his fellow officers are Eternals who feed off the thoughts and emotions of their kidnapped human crew – Ephemerals – in order to fill their own empty existences.
The Master forces Kamelion to pose as King John in 13th century England.
All five Doctors (Peter Davison, Patrick Troughton, Jon Pertwee, Richard Hurndall and Tom Baker) and many of their old companions are taken out of time and deposited in the Death Zone on Gallifrey. There they must battle not only the Master, but Daleks, Cybermen and Yeti in order to reach the Dark Tower and discover the Tomb of Rassilon.