The Doctor and Mel land in what appears to be an orbiting warehouse, a delivery facility with a dangerously erratic computer.

Whilst Mel is helping with repairs, the Doctor begins to realize that not everything in the warehouse is as it seems. Why do no goods ever seem to leave the shelves? Why are the staff so obsessed with the stocktake? And who is the mysterious Supervisor?

On the planet below, the Doctor discovers that the computer might be the least of their problems – and that they should be more concerned with the space station's mould and vermin...