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 meanwhile has its instruments jammed by a mysterious signal and is forced to materialise on board a massive spaceship in a fixed orbit. The Doctor discovers that the signal - a beam to guide the ship's transmat capsule - is being transmitted from Earth. He travels down to the planet in the capsule, leaving Nyssa and Tegan in the TARDIS with the co-ordinates pre-set to follow.

Things go wrong, however, as the Doctor arrives in 1983 but the TARDIS materialises in 1977. Tegan and Nyssa encounter a man with a badly burned body and think that this could be the Doctor.

Tegan goes for medical aid and runs into the Doctor's old friend the Brigadier, now retired from UNIT and working as a maths teacher at the school. They join Nyssa and, at the mysterious stranger's urging, use the TARDIS to travel up to the spaceship.

The stranger is actually Mawdryn, one of a group of alien mutants travelling endlessly in a state of perpetual regeneration brought upon themselves through the use of a stolen Time Lord device.

The Doctor takes Turlough and the 1983 version of the Brigadier up to the ship in the transmat capsule. He reluctantly agrees to supply from his own body the energy needed to end the mutants' ordeal, even though this will mean the loss of his remaining regenerations.

In the event, however, the energy comes not from the Doctor but from an explosion caused when the two Brigadiers meet.

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 liner docks with what appears to be a hulk floating in space. This is Terminus, which claims to offer a cure for lazar disease. It is crewed by a group of armoured slave workers, the Vanir, while the cure is administered by a huge dog-like creature known as the Garm. Nyssa, who has contracted the disease from sufferers transported aboard the liner, discovers that the cure - involving exposure to radiation - does actually work. T

he Doctor and Kari meanwhile learn that the ship, once capable of time travel, was responsible for the creation of the universe when an ejection of fuel from one of its engines caused the 'big bang'. Aided by Kari and the Garm, the Doctor is able to disconnect a still active but damaged engine that is on the point of exploding - something that could result in the universe's destruction.

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.

Turlough attempts to escape the Black Guardian's influence by jumping into space but is rescued and taken on board the ship of Captain Wrack - another of the Eternals. Two of the other ships in the race are destroyed and Turlough discovers that this has been brought about by Wrack, using a concentrated beam of mental energy with the aid of the Black Guardian.

The Doctor boards Wrack's ship, finds her source of power and, with Turlough's help, ejects her and her number two, Mansell, into space. The Doctor and Turlough then pilot the ship into port - a glowing crystalline structure hanging in space - and, in doing so, win the race. The White Guardian offers a portion of Enlightenment to Turlough, while the Black Guardian demands that the boy give the Doctor over to him in exchange for a huge diamond within a glowing artifact - apparently the prize.

Turlough makes his choice: he sweeps the crystal from the table straight toward the Black Guardian, who vanishes in flames. The boy is now free, as Enlightenment was not in fact the crystal but the choice.