The power to save Mawdryn and the others from their undead existence and to cure Tegan and Nyssa of the viral equivalent that rendered time travel impossible eventually comes from the Blinovitch limitation effect, although a line of dialogue indicates that the TARDIS had a role in this as well. Mawdryn briefly has the fourth Doctor's red coat over his shoulders.The Doctor's conversation with Lethbridge Stewart, and subsequent flashback sequence, covers Jo Grant, Sarah Jane, Liz Shaw, Yeti, Cybermen, Axons, Daleks, robot K1, Zygons, and the Brigadier's previous rank as Colonel.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.
When talking to Turlough in the TARDIS in the fourth episode the Black Guardian appears to spit on himself.
The Doctor refuses, explaining to Tegan that if he does so it will be the end of him as a Time Lord. He enters into a pact with the Black Guardian, seen for the first time since The Armageddon Factor.
The Doctor, Nyssa and Tegan deliver the 1983 version of the Brigadier back home. The Brigadier talks of 30 years of soldiering, and has a photograph of himself from his UNIT days. Benton left the army in 1979 to sell second hand cars, and that Harry Sullivan had been seconded to NATO and was last heard of doing something 'hush hush' at Porton Down.
Mawdryn stole a metamorphic symbiosis regenerator, used by Time Lords in cases of acute regenerative crisis, but it induced a perpetual, deathless mutation.
He and his fellow scientists were exiled from their homeworld, but their research could come up with no cure.