At a top-secret UNIT facility Dr Sally Arnold is studying an alien artifact, a Nestene energy unit. It is unresponsive until, as a last resort, she feeds into it a cosmic signal from UNIT's most powerful deep space scanning satellite. This causes a violent release of energy that kills her assistant Janice. The energy unit has vanished, however it is not gone. The energy unit has made a replica of the warehouse's senior archivist Graham Winslet. A UNIT containment team led by the mysterious Lockwood is dispatched to the warehouse - but this storage facility soon turns into a battlefield as several dormant s stored there are reactivated. Can UNIT stop them? Could there be something of use to them stored in the warehouse?