During World War II, Navy Lt. Cmdr. John Lawrence, Richard Widmark, a strict disciplinarian, is put in 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 the professionally aloof Lawrence, and the relationship immediately takes a turn for the worse when they brawl with sailors aboard their transport ship. The ship's captain, Lt. Cmdr. Pete Vincent, Gary Merrill, understands the natural resentment the elite UDT men feel over the death of Cassidy, which they have transferred to Lawrence, and offers to go easy on the team at captain's mast. The by-the-book Lawrence, however, elects to hold his own mast and disciplines the entire team just before a dangerous reconnaissance mission to ascertain the safest landing beach during an upcoming invasion of a Japanese-held island. Lawrence is scornfully perceived as afraid when he splits up the platoon and puts team executive officer Lt. Klinger, Robert Patten, 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 pick-up boat receives a direct hit from artillery during pick-up operations, killing Klinger and most of his men. Lawrence sees that two frogmen, including Chief Jake Flannigan, Dana Andrews, are still in the water, but rather than risk loss of the information already gathered, orders a rescue boat launched and continues back to the transport. The rescue succeeds in recovering the two swimmers, but Lawrence's apparently cowardly action increases the unit's ill will toward him. An embittered 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 new landing site for the invasion.
The next morning, Lawrence, who is sick with coral poisoning, does not reveal his illness when he puts Flannigan in charge of the mission and stays behind. Convinced now that Lawrence is a coward, the men angrily but efficiently complete their task, although 'Pappy' Creighton, Jeffrey Hunter, 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 pick-up boat. Back on the ship, Creighton is put 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 for giving in to the prank, and soon 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. Soon after, Lawrence receives orders to blow up 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 soon Lawrence is recuperating beside Creighton. Finally won over by Lawrence's bravery, the men show their acceptance of him by asking him to sign the portrait they have drawn of Cassidy to present to his widow.
