A World War II-era German submarine missing for 20 years is retrieved in the Bahamas by diver Mark Brittain, hired by the wealthy Rosa Lucchesi and her partner, Vic Rossiter, who have been searching for Spanish galleons. The recovery of the sub results in a plot devised by Eric Lauffnauer, a U-boat officer during the war, to pull a daring million-dollar heist on the British ocean liner Queen Mary, which he and the others plan to rob on the high seas while the liner is making a transatlantic crossing. Brittain gets the sub in working order with the assistance of his own partner, Linc, and a new man, Moreno, a war hero and expert with engines. Disguised as officers from a British vessel on a top-secret mission, Brittain, Rossiter and Lauffnauer board the Queen Mary, where they seize the bullion in the cargo hold. The captain complies after the pirates threaten to open fire on the ship and its civilian passengers. Rossiter's greed leads to his being killed by a member of the Queen's crew. Brittain must abandon the money when Lauffnauer prepares to dive the sub without him. A U.S. Coast Guard cutter in the vicinity comes to the ocean liner's aid. Lauffnauer elects to fire the sub's torpedoes at it. When the others protest, he pulls a gun. Rosa tries to stop him and Lauffnauer accidentally shoots his friend, Moreno. The Coast Guard cutter destroys the torpedoes that Lauffnauer manages to fire from the U-boat. Brittain, Rosa and Linc dive off the sub, just before it is rammed by the Americans. They survive, paddling a raft, but their mission has resulted in three deaths and netted them nothing.