And we reach Logopolis in our Doctor Who marathon.

There’s a bit in this story. The Doctor and Adric return to Earth to recallibrate the Tardis against a real Police Box. The are tailed by the mysterious Watcher. The Master encapsulates the Doctor’s Tardis with his own.
It’s Tegan’s first episode: her car breaks down, she tries to use the Police Box, the Master uses his tissue compressor on her aunt, and she ends up accompanying 4 and Adric to Logopolis to take advantage of their Block Transfer Computation. The Watcher brings Nyssa to Logopolis.
The Logopolitans have been trying to hold the collapse of the universe at bay, which was the cause of the Charged Vacuum Emboitments that drove much of the plot in this season.
The Master’s dastardry imperils the universe, he and the Doctor have to work together to solve the problem, with The Watcher whisking the three young’uns to safety.
The bad guy can’t help himself and has to do one more betrayal, and the 4th Doctor dies of natural causes.

The Watcher turns out to have been a censored version of the 5th Doctor, existing to assist while the 4th deals with the main threat, and they merge as the regeneration took place. As a lad, I didn’t not understand what was going on at all. (I suppose it is similar to Cho-Je being aided by his future version in the previous regeneration story, Planet Of The Spiders.) Nonetheless it was very impactful, no pun intended, and Logopolis is one of the serials frequently referenced by future Doctor Who stories.

DOCTOR: “Your aunt? Woman in the white hat, red sports car?”
TEGAN: “You’ve seen her?”
DOCTOR: “Well, a little of her.”