Both teams are being aided by an alien race called the Kulan. They were stranded on planet Earth after their scout ship crashed there and, as far as the rival teams are concerned, the Kulan are motivated by a desire to get back to their home planet.
What they don't realize is that in fact the wrecked scout ship was part of a Kulan invasion force, sent ahead of the rest of its fleet to investigate Earth. The Kulan are a ruthless race who invade planets to exploit whatever economic value they contain, and the rest of the fleet is due to arrive in 2001.
However, the stranded Kulan need to escape planet Earth's radio-thick atmosphere to contact the fleet and arrange their rendezvous. Will one of the teams succeed in helping the Kulan to destroy Earth -- or will the Doctor succeed in averting a catastrophic attack?