The TARDIS lands at the site of a hyperspatial collision between two spacecraft - as a result of which, neither ship is dimensionally stable, risking the lives of all those aboard. The Doctor, K-9 and Romana offer to help out. When a crewmember is found dead, his face lacerated by huge claws, it seems something deadly has been released by the accident. But how is the death linked to the discovery that the killer drug Vraxoin has been smuggled aboard?

The answer could be the savage Mandrels, hideous swamp-creatures from the planet Eden, which soon tear through the corridors of the helpless spaceships...