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.