During a North Sea military exercise, the crew of the destroyer HMS Nemesis detect what they suspect is a submarine following them. But it’s actually a futuristic ship with an alien occupant, Arianda.

The Doctor and UNIT are called in, but things are already running out of control. The damaged craft is leaking particles and contaminating the Nemesis with time disruption.

But that’s not the biggest problem. For Arianda is being followed by the warships of the Horofax, who have picked precisely this moment to invade. Soon the destruction of humanity’s future will begin.