Simon interrupts an armed robbery and is told by Teal that the gang in question are are building up to a last big job.
On Zolfa-Thura, Meglos plans to steal the ancient and powerful Dodecahedron from the neighbouring world of Tigella. But first, the Doctor must be prevented from interfering.
Masquerading as the Doctor, Meglos penetrates the underground city of Tigella. The Doctor and Romana are trapped in an ever repeating circle of time and powerless to act.
The Doctor must convince the Tigellans of his innocence if he is to prevent Meglos from using the dodecahedron to destroy their world.
While Meglos escapes with the Dodecahedron, the Doctor is mistakenly held responsible for the theft and is sentenced to a gruesome death as punishment for his crime.
