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 in charge of a diversion to the more dangerous beach, where the main landing is scheduled.