The TARDIS lands at the site of a hyperspatial collision between two spacecraft, the Empress and the Hecate. As a result, neither ship is dimensionally stable, risking the lives of all those aboard. The Doctor, K9 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 - and where has the supply come from?The Doctor soon realises that the aliens stored in a projection machine may hold the answers - and that the cages holding these specimens are anything but secure as the savage Mandrels, hideous swamp-creatures from the planet Eden, tear through the corridors of the helpless spaceships...