The Doctor finds himself drafted in to examine some bizarre new drugs that claim to enhance the natural potential of the competing athletes. But what is their real purpose? Why are members of the Security Forces disappearing randomly? And just why is Chase Carrington, manufacturer of the drug, so protective of his company's secrets?

Watching and waiting, at the very heart of Micawber's World, are a race of parasites the Doctor has fought before. The Wirrrn have come to the Milky Way from Andromeda, determined to spread their seed throughout a whole new galaxy, and it seems to Sam that the Doctor's hands are too full to pay their threat full attention.

This is another in the series of adventures featuring the 8th Doctor and Sam which reintroduces the Wirrrn from the 1975 television adventure The Ark in Space.