Operation Pacific

During World War II, the Thunderfish is assigned a special mission to rescue a group of nuns and children, including a newborn nicknamed 'Butch', and to transport them to Pearl Harbor. The crew must navigate dangerous waters and plan a careful evacuation under the threat of enemy patrols. The objective sets the tone for a tense voyage into a war-torn theater.

The Thunderfish spots a Japanese aircraft carrier and makes an attack, but its torpedoes fail to explode, exploding halfway to the target. The carrier's escorting destroyers press in, forcing the submarine to fight its way free. The crew holds steady as they escape the pursuing surface forces.

Back at Pearl Harbor, Executive Officer Duke Gifford visits Butch at the base hospital and runs into his ex-wife, Mary Stuart, a Navy nurse. They share a passionate kiss, reigniting old feelings, while Mary begins a relationship with Duke's younger brother Bob Perry. Duke, torn between duty and love, pursues her but is quickly sent back to sea.

Upon returning to sea, Duke continues his pursuit of Mary but remains constrained by duty. The ship heads out again with the war escalating around them. The personal drama lingers while the Thunderfish carries out its assignments.

During a subsequent patrol, the Thunderfish spots a Japanese freighter only to have its torpedoes fail again. The freighter turns out to be a heavily armed Q-ship that opens fire when they close in. Mortally wounded, Commander Perry orders a crash dive, and Duke orders a battle surface, ramming the Q-ship to sink it.

Thunderfish limps back with a damaged bow, heading for repairs at Pearl Harbor. The crew reflects on the fragility of life under the ocean and prepares for the next mission. The bow is secured as the ship comes back into port.

Duke teams with torpedo specialists to uncover why their torpedoes won't explode, and the mystery is finally solved. He returns to discuss the breakthrough, intending to celebrate with Mary, but she rejects him, choosing her career and memory over a new romance. Commander Steele overhears and admonishes Mary for sacrificing happiness for duty.

Thunderfish resumes duty as part of a scouting line to locate a Japanese fleet heading for Leyte to attack the American invasion force. The crew risks exposure by broadcasting the fleet's position to aid the Allied defense. The revelation helps the Americans prepare for the critical Leyte invasion.

Thunderfish encounters the Japanese fleet and launches torpedoes, throwing the Japanese ships into chaos. They withstand depth-charge attacks and, through determination, manage to sink a Japanese aircraft carrier. The battle rages as the fleet's ships respond to the Thunderfish's bold move.

With American carrier aircraft arriving, Thunderfish shifts to lifeguard duties, rescuing downed pilots under continued attack from Japanese fighters. In the action, the Chief of the Boat and a Junior seaman are killed, and Duke is wounded by a strafing Zero. The crew bears heavy losses as they press on with missions.

Returning to Pearl Harbor after the patrol, Mary is waiting for Duke, and the two reconcile. They head to the hospital with plans to adopt Butch, signaling a possible return to normal life after the war's chaos. The Thunderfish's crew looks toward a future beyond combat.