Australia, 1967. The Doctor, Ace and Group Captain Gilmore's Counter-Measures Group investigate after members Allison Williams and Rachel Jensen discover an alien spaceship at a nuclear test site.
Meanwhile, in 2029, Group Captain Gilmore is found in space, in suspended animation…
Back in 1967, Gilmore's assistant, Delafield, kills a strange snake-like entity attached to civil servant Pendry – killing Pendry in the process. When a severe storm hits the ICMG's camp, Delafield and Jonquil Sharrow kidnap Allison and Rachel, leaving the others behind – but the Doctor engineers a speedy escape. With Ace's assistance, he makes contact with the "snakes" – in reality, friendly aliens on the hunt for deadly-but-beautiful adversaries who have secretly engineered human history of their own ends for millennia.
Now, with Allison and Rachel in danger, can they and their allies prevent a global nuclear disaster?
