An escaped World War 2 Nazi doctor impersonates a murdered English doctor so he can work on a vaccination to protect the Germans in their planned germ warfare.

Lots of slogans such as "Be like Dad, Keep Mum" and "Keep it under your Hat" are visible on the walls in various scenes to reinforce the plot of this British wartime movie illustrating how gossipy talk can result in unknowingly giving valuable information to Nazi spies.

In 1940 Sally Maitland is forced to leave England, ostracised as a Nazi sympathiser by everyone including her well-to-do family. On the ship to Halifax, Canada, she is courted by Polish aristocrat Jan Orloch and by awkward British navel intelligence officer Jimmy Garrick. She shows herself somewhat more amenable to Orloch's advances, and agrees to meet his mother in Halifax. Once there however, it becomes clear no-one is really who they say they are.