At a boy's school in England, Ibbotson admires the maths teacher's classic car. His smug red-haired friend, Turlough, persuades him to drive the car to end of the drive. Turlough, however continues out into the English countryside and crashes.
He awakes in a vortex with a strange being clad in black looming beside him. Turlough admits that he hates Earth and would do anything to leave.
Below the, the boys begin to recover as the math teacher sadly inspect his wrecked car. The teacher is Brigadier Lethbridge-Stewart, now retired. The guardian tells Turlough he must kill one of the most evil creatures in the Universe - the Doctor. Turlough reluctantly agrees.
Tegan speaks to the Doctor about their adventure on Manussa. He explains that the Mara could only be destroyed during its becoming and tries to persuade her that she will recover. Tegan wants to return to Earth. Suddenly, the TARDIS jerks from a warp ellipse cutout - they are about to collide with an object in a fixed orbit.
Turlough is taken to the infirmary, where he finds a strange crystal next to his bed. The Headmaster confronts him - this is the latest in a series of problems. When he is again alone, the crystal begins to glow and he hears the voice of the Guardian warning that he will not leave until the Doctor is killed.
On the TARDIS, Nyssa sees a ship closing in on them. The Doctor quickly materializes on the approaching ship.
The Brigadier speaks to the Headmaster about Turlough. The Headmaster claims that Turlough's parents are dead - he deals with a strange solicitor in London.
Turlough, on instructions from the Guardian, leaves the infirmary for the hills. Ibbotson follows him.
The TARDIS crew explore the ship. They find it to be an empty gothic luxury ship that has been in perpetual orbit for 3000 years. They find a transmat capsule was taken to Earth six years previously.
Instructed by the Guardian, Turlough activates a control inside a stone obelisk on the hilltop. A transmat capsule appears. Turlough climbs in and takes off. The capsule reappears on the spaceship. Turlough wants to use the ship to return home, but the Guardian insists that he kill the Doctor first.
Ibbotson returns to the school and explains what he has seen to the Brigadier. The soldier follows him up the hillside, insisting solid objects just don't "dematerialize".
The Doctor finds he is unable to take off. He returns to the ship and finds that the capsule has returned and its signal is jamming the TARDIS. He returns to the TARDIS to find Turlough inside the console room.
The Doctor decides to return to Earth in the transmat capsule to repair the fault. The TARDIS will automatically follow. Turlough insists on accompanying him.
The Doctor opens the obelisk and begins to repair the transmat machinery. The Black Guardian orders Turlough to kill him. Turlough grabs a massive stone and raises it over his head, ready to strike down the unsuspecting Time Lord...