Castrovalva

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.

• Protreeve/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);


• UNCREDITED CAST:
• 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;