Under the supervision of the Doctor, the Brigadier and Dr Liz Shaw, UNIT are getting ready to activate Hotspur: their new, advanced early warning system.

But something goes wrong. Can it be that UNIT has been betrayed from within? Suddenly bases are falling across the globe, and only the Doctor and his friends are able to escape.

Not knowing how far the conspiracy goes, the Doctor, Liz and the Brigadier become fugitives. Their investigations lead them to the Fulcrum military training facility. And something beyond the Earth.