Deep space, 5000AD. An Earth supply shuttle is infected by a strange sentient space-borne virus, which quickly spreads to the crew of the refuelling station on the moon Titan. Even worse, the Doctor himself falls victim. But why is Leela immune, and what is the virus's ultimate goal?

Leela and Lowe take the Doctor to the Bi-Al Foundation for help. But how can they cure an infection of the mind?

The Nucleus, now grown to human size, is determined to spawn and take over the Galaxy. Only the Doctor and Leela can stop it.