The Doctor is invited by the leader of Tigella, Zastor, to visit the planet because there is concern between the religious Deons and the scientific Savants. The TARDIS is intercepted by Meglos, the megalomaniacal last survivor from the planet Zolfa-Thura.
The TARDIS is trapped in a time loop, although the Doctor manages to break free. Meglos uses the stolen body of an Earthling and transforms himself into a duplicate of the Doctor.
Disguised as the Doctor Meglos steals the Tigellans' power source - a mysterious dodecahedron. Meglos plans to use the dodecahedron to power an apocalyptic device which would be used to destroy Tigella.
The Doctor is hindered by the Gaztak, hired by mercenaries, but manages to succeed in tampering with the dodecahedron, and the resulting explosion destroys Meglos.
- Voice of K-9: John Leeson
- Romana: Lalla Ward
- General Grugger: Bill Fraser
- Lieutenant Brotadac: Frederick Treves
- Zastor: Edward Underdown
- Lexa: Jacqueline Hill
- Caris: Colette Gleeson
- Deedrix: Crawford Logan
- Earthling: Christopher Owen
- Tigellan Guard: Simon Shaw
- Meglos: Tom Baker, Christopher Owen
- Gaztacs: Tony Allef, Bruce Callender, Hi Ching, John Holland, James Muir, Ranjit Nakara
- Tigellans: Michael Gordon Browne, Harry Fielder, Pater Gates Fleming, Laurie Goode, Geoff Whitestone
- Ceremonial Guards: Michael Brydon (4), David Cleeve (4)
- Savants: (all 2) Howard Barnes, David Cole, Stephen Kane, John Laing
- Lexa's Deons: all 1,3,4) Terence Creasey, Ray Knight, Chris Marks, Eddie Sommer
- Deon Elders: (all 1,3,4) Lewis Hooper, Sylvia Mariott, Stephen Nagy
|BBC 1||530. Part One||Saturday, September 27, 1980 6:15 PM - 6:40 PM||5.0M||61%|
|BBC 1||531. Part Two||Saturday, October 4, 1980 6:15 PM - 6:40 PM||4.2M||64%|
|BBC 1||532. Part Three||Saturday, October 11, 1980 5:45 PM - 6:10 PM||4.7M|
|BBC 1||533. Part Four||Saturday, October 18, 1980 5:40 PM - 6:05 PM||4.7M||63%|
- Television Centre Studio 3
- Television Centre Studio 6
This story is notable for the appearance of Jacqueline Hill, some seventeen years after she played one of the first companions, Barbara Wright.