George Blake (George Formby) is desperate to join the navy and to do his bit in the fight against Hitler. The trouble is, the navy doesn't want him! It looks like George will see out the war as a steward at the Senior Service Club - until fate takes a hand! During an air raid blackout, he accidentally puts on the naval uniform of one of his chums - and before he knows it, everyone thinks he's a fully-fledged matelot. Now, George must keep up the pretence, because he's terrified of being considered a German spy! Meanwhile, real German spies are desperate to sink a new U-boat hunting warship and when George blunders across their cunning plans he finds himself in even hotter water!