Henry Bulman, a retired submariner from Her Majesty's Royal Navy, reactivates an abandoned Nazi U-boat and discovers a forgotten cache of deadly nerve gas. Taking advantage of his "good luck," Bulman decides to extort $20 million from the United States by threatening the lives of some 30,000 Americans. Assigned to stop Bulman or call his bluff, Steve dives to the submarine, bionically resists the water pressure, and boards the craft. But he is taken prisoner and then is faced with making a horrifying decision.