The Doctor has an idea that may repair the chameleon circuit. For this the Doctor needs the exact measurements of a police box, and there is one on the Barnett Bypass in 1981. But before the Doctor can materialise around it, someone else already has done.
In the same lay-by two women have car trouble. While Tegan enters the police box, thinking it is real, someone else has emerged, and Tegan's aunt has been shrunk. In order to get rid of whatever has entered the TARDIS, the Doctor plans to materialise underwater and open the doors. The TARDIS misses, and materialises aboard a ship, where Doctor meets the Watcher.
With the measurements for a police box the Doctor materialises the TARDIS on Logopolis. The Logopolitans have the necessary skills to recalculate the block transfer computation, but the calculations are interfered with, and when applied the TARDIS shrinks.
The Watcher has also sent for Nyssa, and the Doctor, Adric, Tegan and Nyssa find that the culprit is the Master, who plans to hold the Universe to ransom. The plan backfires and entropy destroys the Traken Union, and Logopolis.
Logopolitans had copied the Pharos project on Earth. To fix the problem the Doctor, his companions, and the Master, travel to Earth. The Doctor manages to halt the entropy, but in a battle with the Master the Doctor falls from a radar dish and triggers his regeneration...
- Adric: Matthew Waterhouse
- Nyssa: Sarah Sutton
- Tegan Jovanka: Janet Fielding
- The Master: Anthony Ainley
- Detective Inspector: Tom Georgeson (1-2)
- Aunt Vanessa: Dolores Whiteman (1)
- The Monitor: John Fraser (2-4)
- Security Guard: Christoper Hurst (4)
- New Doctor: Peter Davison (4)
- The Watcher: Adrian Gibbs
- Policemen: (all 1-2) Ray Knight, Peter Roy, Derek Suthern
- Logopolitans: George Ballantine, Douglas Bather, Jim Delaney, Billy Gray, Jimmy Mac, Brynchan Powell, Terry Randle, Evan Ross, Roy Seeley, Charles Stewart, Colin Thomas, John Tucker, Walter Turner, Peter Whitaker, Bill Whitehead
- Pharos Technician: Robin Squire (4)
|BBC 1||550. Part One||Saturday, February 28, 1981 5:10 PM - 5:35 PM||7.1M|
|BBC 1||551. Part Two||Saturday, March 7, 1981 5:10 PM - 5:35 PM||7.7M||61%|
|BBC 1||552. Part Three||Saturday, March 14, 1981 5:10 PM - 5:35 PM||5.8M|
|BBC 1||553. Part Four||Saturday, March 21, 1981 5:10 PM - 5:35 PM||6.1M||65%|
Repeat Broadcasts on the BBC
|BBC 2||Part One 1||Monday, November 30, 1981 5:40 PM - 6:05 PM||5.5M|
|BBC 2||Part Two 1||Tuesday, December 1, 1981 5:40 PM - 6:05 PM||5.0M|
|BBC 2||Part Three 1||Wednesday, December 2, 1981 5:40 PM - 6:05 PM||6.0M|
|BBC 2||Part Four 1||Thursday, December 3, 1981 5:40 PM - 6:05 PM||5.4M|
- Television Centre Studio 3
- Television Centre Studio 6
- A40 lay-by, Barnet bypass, Denham, Bucks
- Albert Bridge, River Thames, London
- Cadogan Pier, River Thames, London
- Caversham, Berks
- Henley-On-Thames, Oxon
Sarah Sutton does not appear in Part One. Although she was credited as a companion in the previous story she doesn't really travel with the Doctor until this adventure.
This story sees the departure of Tom Baker.
The story features the first extensive flashback sequence (in Part Four). The clips are: The Master (The Deadly Assassin 1), a Dalek (Destiny of the Daleks 4), The Pirate Captain (The Pirate Planet 2), the Cyberleader (Revenge of the Cybermen 3), Davros (Genesis of the Daleks 5), a Sontaran (The Invasion of Time 5), a Zygon (Terror of the Zygons 3), the Black Guardian (The Armageddon Factor 6), Sarah (Terror of the Zygons 2), Harry (The Sontaran Experiment 2), the Brigadier (Invasion of the Dinosaurs 2), Leela (Robots of Death 1), K-9 (The Armageddon factor 2), the first Romana (The Stones of Blood 1) and the second Romana (Full Circle 1).