Castrovalva

The 5th Doctor is having a rough regeneration and needs to spend time isolated from outside signals. The Master captures Adric to build a trap, while Tegan and Nyssa try to keep the Doctor safe. Escherian geometry ensues.

I’ve seen this one several times and know it well, particularly remembering the impossible collapsing geometry of Castrovalva, the threat of the Big Bang, the successive clues about the trap. Recursion is a theme.
The 5th Doctor does some brief mimicry of the 1st and 2nd doctors as he wanders around.

They really did have Tony Ainley laugh a lot.

TEGAN: “What’s a Zero Room anyway? The Doctor said something about null interfaces.”
NYSSA: “ I suppose it’s some sort of neutral environment. An isolated space cut off from the rest of the universe.”
TEGAN: “He should’ve told me that’s what he wanted. I could’ve shown him Brisbane.”