Parts: 4
Writer: Christopher H Bidmead
Airdates: 4 January 1982 - 12 January 1982
Production Code: 5Z

Doctor: fifth
Companions: Adric, Nyssa, Tegan

Megalomaniac of the Week: the Master

Story: Following the Doctor's regeneration, Nyssa and Tegan race to get the dazed Time Lord back to the TARDIS as Adric distracts the local constabulary. But then the Master's TARDIS materializes and knocks out the police, and when Nyssa and Tegan manage to find Adric again, he has begun to act strangely.

After setting the co-ordinates, Adric follows the Doctor into the maze of TARDIS corridors...and disappears. Nyssa and Tegan finally go after the Doctor and help him to find the Zero Room, a pinkish-grey chamber with great healing properties that will ease the Doctor through his post-regenerative trauma. But they soon realize that the TARDIS has begun to heat up, and when Nyssa investigates, she finds that the ship has been sent on a course directly back to the very beginning of the universe...

The Master has captured Adric and is using him to destroy the TARDIS crew. Escaping from his trap only comes at great cost to the TARDIS and the Doctor's safety, so Nyssa and Tegan decide that it would be wise to take the Doctor to Castrovalva - one of the Dwellings of Simplicity. But the Master has another trick up his sleeve - a recursive space-time trap that will catch the TARDIS crew unaware...