On holiday in Lanzarote, a young American girl named Peri narrowly escapes drowning when she is rescued from the sea by Turlough and taken into the TARDIS to recover. The Doctor is on the island because the TARDIS has detected a mysterious signal being transmitted from an unknown artifact retrieved from the sea bed by Peri's stepfather, Howard Foster.

The Master reasserts his control over Kamelion and gets it to bring the TARDIS, along with the Doctor, Turlough and Peri, to the planet Sarn, where he is hoping to use that world's supply of revitalising numismaton gas to restore his body - accidentally shrunken in an experiment with his tissue compression eliminator weapon - to its correct size.

It transpires that amongst the Sarn natives, who worship a fire god named Logar, are political prisoners from Trion - Turlough's home world. Turlough too is revealed to be a political refugee. He meets his brother and later, when a spaceship arrives from his home world, discovers that Trion has granted an amnesty to all political prisoners.

The Master is apparently killed when a stream of numismaton gas in which he is bathing turns to a normal hot flame. The Doctor destroys Kamelion at the robot's own bidding as it has become completely unstable. Turlough leaves to return to Trion, while Peri goes with the Doctor.