The Doctor and Jo arrive on the planet Far. The Doctor wants to attend the grand opening of one of a Hyper Gateway. But they land in the wrong time period, long after the Gateway has been in service, and the Daleks have conquered Far.
During a military exercise, the crew of the destroyer HMS Nemesis detect what they suspect is a submarine. But it's actually a ship with an alien occupant, Arianda. The Doctor and UNIT are called in, but things are out of control.