The TARDIS materialises in the middle of the sea on the ocean planet Lethe. The Doctor and Nyssa are rescued by the crew of the Lung of Heaven, captained by Katr Korzen. The planet is ravaged by fractures in the time fields, and the ship is navigated by Constantine, who is a time-sensitive. The ship is on a mission to find the Parliament of Rats, a legendary ship, so that Brunner, the physician-priest, can exorcise it of the White God.
The Doctor is suffering headaches due to the time fractures. Nyssa worries that his recent regeneration has hurt him badly.
The Lung encounters the Parliament of Rats. The crew of the Parliament are disfigured from their travels in the time fractures. They board the Lung, where Korzen has ordered Harbou, the mate, to surrender. Korzen and the Doctor hide below decks because the captain of the Parliament is looking for her.
The crew of the Parliament take Nyssa and Brunner to the Parliament to sacrifice them to their god. Brunner is happy to go, as it will give him a chance to expose the White God as a myth. They are locked in a room with a mirror. Something white comes out of the mirror, and Brunner tries to kill it. Nyssa thinks it is a white hole, but suddenly the Doctor appears to rescue them. He stays behind while Constantine takes Nyssa and Brunner back to the Lung.
The Doctor communicates with the white force and discovers that it is something left behind by the Time Lords. He orders it to destroy itself; the Parliament breaks apart with the Doctor still aboard and he ends up in the sea. Constantine rescues him and drags him to the Lung. When he realises that she was the white power, she disappears. He is rescued by the Lung, where the real Constantine is.
By the time the Doctor and Nyssa leave the ship, Constantine has disappeared.
- The ocean planet is named Lethe; the Doctor points out that Earth mythology includes a story about a river Lethe.
- Korzen captains the Lung of Heaven, while Pym captains the Parliament of Rats.
- The White God, a sea deity, was supposedly captured by the Parliament.
to be added