The Doctor is perturbed when a spate of deaths follows the break-up of an alien mass in the atmosphere. but this is merely the leatest incident in a sinister conspiracy tht threatens the entire planet, and the Doctor himself is embroiled in the plans of all the players. 

The Brigadier's concern is heightened by the possibility of traitors at the very heart of UNIT. Leaving for Geneva to discover the truth, he little realises the deadly motives of an enemy agent on his own doorstep. The Doctor and Liz, meanwhile, discover that London doesn't have a monopoly on alien invasions. 

What are the gargoyle-like creatures that kill without mercy? What do they want from our planet - and how do they figure in top-secret governmental plans? 

As the lines between allies and enemies begin to blur, the Doctor finds himself fighting to save the Earth once again. But who will he be saving it for? 

Featuring the third Doctor, Liz Shaw and UNIT, this adventure takes place between the TV stories INFERNO and TERROR OF THE AUTONS.