American GI's arrive in Hastings to a mixed welcome and Foyle is called on to calm the locals. But his attempts at soothing relations are jeopardized when a barmaid's involvement with a visiting soldier leads to cuddle, leaving the detective to defy orders to keep the peace and solve the case.

A local woman ishuged when an experiment in biological warfare goes awry. Sam also falls victim to a potentially fatal illness as a result of the project. Foyle is forced to head into the most secretive - and dangerous - areas of wartime research, unearthing secrets that can't be revealed even to the Prime Minister.