After his fall from a radar dish the Doctor's fourth regeneration is witnessed by his three companions. The new Doctor, Tegan and Nyssa are separated from Adric, who is kidnapped by the Master. The Doctor's regeneration is unstable, and he needs to spend a lot of time in the Zero room to recuperate.
Meanwhile the TARDIS heads towards Event One, the explosion that created the Universe, and the Doctor only just manages to rescue the TARDIS. Tegan then thinks she had landed the TARDIS on the lush world of Castrovalva.
Castrovalva is not what it first seems: The whole world is a Block Transfer Computation created by Adric on the orders of the Master. The Doctor discovers that the world seems to have no real history, only a fictitious one.
One of the natives causes the destruction of the web which is holding Adric, causing the collapse of Castrovalva. The Doctor and his three companions escape as the city disappears; there is no sign of the Master.
- Adric: Matthew Waterhouse
- Nyssa: Sarah Sutton
- Tegan Jovanka: Janet Fielding
- Portreeve/The Master: Anthony Ainley (Portreeve credited to Neil Tonnay for 3)
- Head of Security: Dallas Cavell (1)
- Ruther: Frank Wylie (2-4)
- Mergrave: Michael Sheard (3-4)
- Shardovan: Derek Waring (3-4)
- Child: Souska John (3)
- The Watcher: Adrian Gibbs (1)
- The Doctor: Tom Baker (1)
- Security Guard: Harry Fielder (1)
- Ambulance Men: (all 1) David Ellis, Derek Matt, Peter Roy
- Double for Shardovan: Ken Barker (4)
- Castrovalvans: Mark Allington, Delphine Auchterlonie, Lyn Austin, George Ballantine, Ilana Barry, David Bulbeck, Jean Channon, Eric Corlett, Colin Cook, Doreen Croft, Sally Gardner, Ernie Goodyear, James Gregory, Steven Griffiths, Dorothy Grumbar, Lucy Gwynn, Dave Harrison, Helen Hembrough, Ray Martin, Giles Melville, Valerie Needham, June Parkhurst, David Payne, Jennie Piper, Malcolm Rose, Rosemary Smith, Maureen Stevens, Cy Town, Mike Vinden, Sheila Vivian, Tina Winter
|BBC 1||554. Part One||Monday, January 4, 1982 6:55 PM - 7:20 PM||10.1M|
|BBC 1||555. Part Two||Tuesday, January 5, 1982 7:00 PM - 7:25 PM||8.7M|
|BBC 1||556. Part Three||Monday, January 11, 1982 6:55 PM - 7:20 PM||10.4M|
|BBC 1||557. Part Four||Tuesday, January 12, 1982 7:05 PM - 7:30 PM||10.5M|
- Television Centre Studio 3
- Television Centre Studio 6
- Buckhurst Park, Withyham, East Sussex
- Crowborough WT Station, Duddleswell, nr. Crowborough, East Sussex
- Harrison's Rocks, Birchden Wood/Attyons Wood, nr. Groombridge, Sussex
Episode 1 : The Doctor's footwear seems to have regenerated with him.
Episode 2 : When Tegan and Nyssa start to carry the zero cabinet (after blowing up the wheelchair in the stream) Nyssa gets her tiara caught on an overhead branch - and it stays up there. Neither Nyssa nor Tegan seem to notice.
Episode 4 : When they're trying to find the way out of Castrovalva, Adric announces "I can see." He then sees the exit to the left - before he's even looked in that direction.
Episode 4 : While the two Castrovalvans are searching for the cause of the broken window there is a scene when they look meaningfully at each other. Just before the camera cuts away the wall behind them starts to collapse.
Part One has a pre-titles sequence consisting of the regeneration from the end of Logopolis, but with new music.
Part One is the first episode of DOCTOR WHO not to be shown on a Saturday (excluding repeats) as the programme is now shown twice a week.
From this story onwards the main character is credited as The Doctor instead of as Doctor Who.
The seventh title sequence makes its debut, again by Sid Sutton. It is a slightly modified version of the one used for Season 18.