At a luxurious vacation house on the Canadian coast, a curious house party has gathered. Its object is not sociability but security - the safety of the crack English engineers whose mission is to design the missile controls for Canada's revitalized navy. One of them has already died mysteriously when those amiable MI5 agents, Patrick and Ingrid (who are also husband and wife), are ordered to join the party.

High-stakes charades is the game of preference, and all the guests take part. But an 'accidental' car crash is followed by an 'accidental' poisoning. And it suddenly seems clear to Patrick and Ingrid that a traitor has crashed the festivities. The very difficult question is who.