When a recently paroled friend of Boston Blackie is killed, he finds himself once again the prime suspect of Police Inspector Farraday.

His sidekick, "the Runt", has to postpone his wedding to statuesque bubble dancer Dixy Rose Blossom.

Meanwhile, crooks Joe Herschel, Sammy Walsh and Marty Beck force their prisoner, Ed Barnaby, to reveal where he hid the diamonds.

Slipping inside undetected, he spies through the keyhole of the door to Herschel's office and sees the crook put the diamonds in his safe.

Unaware that Herschel double crossed them and had the real diamonds, Walsh and Beck exchange Betty for the fakes.

Herschel returns too soon and exposes the fakes, but Blackie and Betty eventually manage to escape, aided by a citywide wartime practice blackout.