A mad scramble for stolen loot ensues after Boston Blackie has prisoners released for work in a wartime defence plant.

Through flashbacks going as far back as the end of WW1, the story of a Nazi war criminal is exposed during his trial.

Sam Gallagher returns home to Los Angeles as an undercover spy for the Navy, getting a job at the shipyards where his brother, Jeff, is a foreman. Jeff still resents Sam for abandoning the family years ago and fears he may steal away Lea Damaron, his current girlfriend -- who is Sam's old flame. While Sam tries to sniff out Nazi saboteurs in the plant, he grows closer to Jill McGann, the agent tasked with pretending to be his wife.

Panic arises among Allied POWs aboard a Japanese freighter when they learn that the ship is actually a decoy target for American submarines on night patrol. The prisoners unite and attack their Japanese captors just as an American sub surfaces and, not knowing the prisoners are aboard, prepares to torpedo the ship.