When a dead Englishman is discovered dumped in Dieppe, a receipt leads Inspector Maurice Sardou to Gladys Moon and her son Trevor. It appears that Gladys, too, has been attacked, and police quickly arrest a student. Trevor believes this is a mistake, as he's sure Gladys's attacker used a toy gun; the murder weapon, however, is soon recovered, and it's real – and lethal – enough.

In England, Trevor links the dead man to alleged corruption by contractors building the Channel Tunnel and talks to the man's colleague, Dunblane, and also to the widow. As a result two heavies take both Gladys and Trevor's girlfriend Cecile hostage, threatening Trevor also before being routed by Cecile. The English police warn Trevor to stay clear of the investigation, but back in France he is challenged by Sardou to help exonerate the student by dowsing for the alleged toy gun.

A witness comes forward who saw the real gun thrown away, and while Sardou and the English police pursue this lead Gladys learns