Synopsis: The Doctor and his new friend Kul are on aquabikes, traveling through what is probably the bloodstream of the planet. They are beset by antibodies who force them to crash their bikes into what turns out to be the ship of Kul's ancestors. On this vessel, Kul's forbearers escaped an unknown catastrophe on their homeworld, bound for some location long since lost to Kul's people.
Exploring the ship, they find that it's been converted to the planet's purposes. Robots once servants of Kul's race now attack Kul and the Doctor. The Doctor eliminates the immediate threat, but then discovers that the ship's computer core has been taken over by what are probably nerve endings leading down into a deep pit.
The Doctor surmises that they must be getting close to the brain of the planet. Judging from the specific damage to the ship, the Doctor reckons the planet needs knowledge. Needing to figure out why the planet is sucking the life out of the computer cores of alien vessels, the Doctor tells Kul—almost certainly because he's desperately worried about the fate of his missing TARDIS—that he needs to go into the pit and find out what the planet "is planning to do—and fast!"