Harbor, Bob Perry believes that Duke's order to dive the boat killed his brother, and he refuses to listen to Duke's explanation. Mary tries to comfort Duke, but he rejects her attempts, declaring he only did his duty and feels no regret. Working with the sub base's torpedo specialists, Duke and the crew of the Thunderfish undertake an investigation to find out why the torpedoes are not exploding. When they finally discover the answer, Duke goes to Mary to celebrate, but she rejects him. Since he wouldn't let her into his life when he was at his lowest, she feels that they cannot have a real relationship. Her superior, Cmdr. Steele (Kathryn Givney), overhears the conversation and castigates Mary for throwing away her chance for happiness with Duke. Once again Thunderfish heads to sea, this time as part of a scouting line searching for a Japanese fleet heading for Leyte to savage the American invasion force there. Thunderfish finds the enemy. Even though it will reveal their presence, Duke broadcasts the fleet's position. Once Pearl Harbor acknowledges the message, Duke salvoes his torpedoes and makes a run for it, throwing the attacking Japanese warships into chaos. Despite enduring a battering from Japanese depth charges, Thunderfish manages to sink a Japanese aircraft carrier. In the next phase of the battle, American carrier aircraft arrive and attack the Japanese fleet. Thunderfish, now assigned to lifeguard duty, helps to rescue shot down American flyers, and does so while under attack from Japanese fighters. While rescuing LT Bob Perry, the Chief of the Boat and Junior, a seaman from a Navy family, are killed and Duke is wounded by a strafing Japanese Zero. When the Thunderfish returns to Pearl Harbor after the patrol, Mary is waiting for Duke. The two, reconciled, head to the hospital, intending to adopt Butch. Cast John Wayne as Lt. Cmdr. Duke E. Gifford Patricia Neal as Lt. (j.g.) Mary Stuart Ward Bond as Cmdr. John T. "Pop" Perry Scott Forbes as Lt. Larry Philip Carey as Lt. (j.g.) Bob Perry Paul Picerni as Jonesy William Campbell as the Talker Kathryn Givney as Cmdr. Steele Martin Milner as Ensign Caldwell Cliff Clark as Commander, SUBPAC Jack Pennick as the Chief Virginia Brissac as Sister Anna Vincent Fotre as Soundman Lewis Martin as Squad Commander Sam Edwards as Junior James Flavin Mick Shore Patrol Commander (uncredited) Harry Lauter Freddie Commanding Officer