'Castrovalva' (4 Parts) Story Code '5Z' by Christopher H. Bidmead

After the Doctor has regenerated, the Master kidnaps Adric. The Doctor, Tegan and Nyssa escape from the Pharos Project guards and return to the TARDIS, where the Doctor, weakened from his traumatic regeneration, retreats to the calming environment of the ship's Zero Room. The Master attempts to send the Doctor's TARDIS back in time to Event One - the hydrogen inrush that created the universe - but the Doctor jettisons a quarter of the TARDIS, including the Zero Room, in order to provide the necessary energy to escape destruction. Nyssa and Tegan look for another peaceful retreat for the Doctor's recuperation, and after finding an entry in the ship's databanks, take the TARDIS to Castrovalva, a citadel of harmony and learning perched high atop a mountainside. Travelling to the city, the three travellers meet Shardovan, its librarian, and the elderly Portreeve, who governs the city, and whose strange tapestry portrays scenes from the past, including the travellers themselves. The Doctor begins to recover, and realises that Castrovalva is far more than it appears - he and his companions are caught in a space / time trap known as recursive occlusion. The Portreeve is in reality the Master, who is holding Adric prisoner in an energy web concealed behind the tapestry. He has forced the young boy to utilise his mathematical skills to create the city with Block Transfer Computations - a technique based on the Logopolitans' work. Realising that he and his fellow citizens are nothing but a fabrication, Shardovan plummets into the web, freeing Adric, but dooming the city to destruction. As the Doctor and his companions escape the city, the Master is dragged back inside by the very people he created, and Castrovalva vanishes.

Peter Davison (The Doctor), Matthew Waterhouse (Adric), Janet Fielding (Tegan), Sarah Sutton (Nyssa), Anthony Ainley (The Master), Dallas Cavell (Head of Security), Frank Wylie (Ruther), Derek Waring (Shardovan), Micheal Sheard (Mergrave), Neil Toynai* (Portreeve), Souska John (Child)
Note:Pseudonym for Anthony Ainley to conceal plot twist

Produced by John Nathan-Turner
Directed by Fiona Cumming

TX:
Part 1 - 4th January, 1982 @ 6.55pm - 7.20pm++
Part 2 - 5th January, 1982 @ 7.00pm - 7.25pm
Part 3 - 11th January, 1982 @ 6.55pm - 7.25pm
Part 4 - 12th January, 1982 @ 7.05pm - 7.30pm

++Part 1 premiered at 3.30 pm on BBC Scotland on 4th January, 1982

Notes:
*Featuring the Fifth Doctor, Adric, Nyssa and Tegan

*There is a pre-title sequence reprising the events leading up to the Doctor's regeneration

*A new title sequence is introduced, again created by Sid Sutton, featuring Peter Davison's face

*For the first time, the Doctor is credited as 'The Doctor' on the closing credits, instead of the previous 'Dr. Who' or 'Doctor Who'

*Working title: 'The Visitor'