The transmat device explodes, throwing Turlough to the ground. The TARDIS appears briefly on the hillside and then vanishes.
The TARDIS appears to materialize in the right place. Tegan and Nyssa find the hillside abandoned.
As the Doctor inspects the device, the Brigadier appears. He does not recognize the Doctor.
As Nyssa and Tegan wait, the capsule appears. Inside is a horribly scared man. When he hears of the TARDIS, he has the girls return him there.
The Brigadier seems to know nothing of the Doctor or the TARDIS. When the Doctor mentions UNIT, they god to his quarters. The Brigadier still does not remember the Doctor. The Doctor asks the Brigadier about Benton and Sullivan and he remembers them. But he has no recollection of Liz Shaw, Sarah Jane or Jo Grant. The Brigadier collapses into a chair and suddenly has a flashback, recalling all of his adventures with UNIT.
The Brigadier, agitated by the Doctor's continuing queries, explains that he had a nervous breakdown some time ago. He now teaches mathematics.
The Doctor mentions that he must find the TARDIS, Nyssa and Tegan. The Brigadier's recognizes Tegan's name -- he met her once.
Tegan goes to the school for help and meets a younger Brigadier. The Doctor realizes that the TARDIS materialized in the wrong time.
Turlough confesses his story to the Headmaster and decides that he won't kill the Doctor. At that, the Headmaster changes into the Black Gaurdian. He dominates Turlough, telling him he will not leave until their bond is honoured.
The Brigadier hears Tegan mention the TARDIS and asks if her injured friend is the Doctor. When he mentions the Queen's silver jubilee, Tegan realizes they are the wrong time.
The Doctor and the Brigadier are informed that Turlough has fled the infirmary again.
Tegan and the Brigadier return to the hillside. Tegan realizes the man they found can not be the Doctor.
Nyssa checks on the Doctor and finds that he is horribly deformed. He wants to return to his ship, even if it means abandoning Tegan. When Nyssa leaves, he reveals that his name is Mawdryn and he was abandoned by the Time Lords.
Nyssa returns with Tegan and the Brigadier. They find that Mawdryn has changed into a horrible alien form, the open top of his skull pulsing with his heartbeat...