The Doctor (Colin Baker) is suffering from some post-regenerative trauma, and, after trying to strangle companion Peri, takes off to a deserted asteroid to become a hermit. There he discovers a plot by the Gastropod Mestor to kidnap two mathematically brilliant twins and shift some planets around in order to release more of his slimy ilk. Obviously he has to be stopped, but is the Doctor up to it? Colin Baker's first outing as the Doctor.