As the voice-over proclaims, the 49th parallel is the only undefended frontier in the world. The parallel is the goal of a small group of Nazis, survivors of a U-boat that has been spotted and sunk in Hudson Bay, deep in Canadian waters. The United States is not yet at war with Germany, and if Corporal Hirth and his six crew members can reach it, they can be repatriated to Germany with no fuss. But their trek across Canada becomes a fierce struggle...

The U.S.S. Sea Tiger is on its last legs until the handsome skipper and his ingenious, if slightly unethical, junior officer scavenge the parts and supplies needed to put the sub back into action. Forced out to sea prematurely by an enemy air attack, the sub leaks and limps along until five stranded Army nurses come aboard and initiate their own renovations. From one mishap to another, the sub ends up a blushing pink, making it a target for both Japanese...