During
World War II, Navy Lt. Cmdr. John Lawrence, a strict disciplinarian, is given charge of Underwater Demolition Team 4 after its former leader, Lt. Cmdr. Jack Cassidy, is killed in action. The unit's men are distrustful of Lawrence and they brawl with sailors aboard their
transport ship. The ship's captain, Lt. Cmdr. Pete Vincent, understands the resentment that the elite UDT men feel over the death of Cassidy, which they have transferred to Lawrence, and offers leniency for the team at
captain's mast. Lawrence holds his own mast and disciplines the team just before a dangerous mission to locate a safe landing beach for an upcoming invasion of a Japanese-held island. Lawrence is scornfully perceived as afraid when he divides the
platoon and places
executive officer Lt. Klinger in charge of a diversion to the more dangerous beach, where the main landing is scheduled. During the mission, Lawrence cuts his leg on
coral, and the diversionary section's transport boat is hit by artillery, killing Klinger and most of his men. Lawrence sees that two frogmen, including Chief Jake Flannigan, are still in the water, but rather than risk loss of the information already gathered, orders a rescue boat and continues back to the transport. The men are saved, but Lawrence's apparently cowardly action increases the ill will toward him. Flannigan and some of the others request transfer to another unit, but Lawrence insists that they first complete the next day's mission to clear the landing site for the invasion. The next morning, Lawrence, who is sick with coral poisoning, does not reveal his illness when he leaves Flannigan in charge of the mission and stays behind. Convinced that Lawrence is a coward, the men angrily but efficiently complete their task, although "Pappy" Creighton, whose brother is a
U.S. Marine, sneaks onto the beach with Flannigan to leave a sign welcoming the Marines. Creighton is shot after the prank, but Flannigan tows him to the pickup boat. Back on the ship, Creighton is in
traction because of the bullets in his spine, and Flannigan confesses to Lawrence that the prank caused Creighton's injuries. Lawrence furiously upbraids Flannigan and all of the men request transfers. While Lawrence is discussing the transfer requests with Vincent, a
torpedo hits the ship but does not detonate. Lawrence volunteers to disarm the torpedo, which has lodged in the
sick bay next to Creighton's bed, and, with Flannigan's help, succeeds. Lawrence receives orders to destroy a Japanese
submarine pen and tells the men that although it will be their last mission together, he is proud to have served with them. Although Flannigan voices disdain that Lawrence will again dodge dangerous duty, Lawrence leads the mission, which is discovered when one of the men accidentally trips a signal wire. Japanese sentries shoot at the men as they plant the charges, and Lawrence is stabbed in hand-to-hand combat with a Japanese diver. He orders Flannigan to leave him behind, but Flannigan tows him to safety. The mission is a success, and Lawrence recuperates beside Creighton. Swayed by Lawrence's bravery, the men now accept him by asking him to sign the portrait they have drawn of Cassidy to present to his widow. ==Cast==