An advance group of Aliens from near the star Arcturus have chosen Britain as their base prior to a full scale world invasion, as it is isolated enough by water to be contained. Their plan: to introduce a chemical into the water supply to turn the people into zombies and slaves. While investigating the break-in at a chemical weapons storage depot, Gunn finds evidence which makes him realize something strange is happening. He discovers the great reservoirs of Wales are the targets, with a badly wanted war criminal as the Aliens' ally.

Location: Wales.