| Director | George Waggner |
|---|---|
| Budget | $1.5 million |
| Box office | $3.9 million |
During World War II, the submarine USS Thunderfish, under the command of Commander John T. "Pop" Perry, while on a special mission to the Philippines, is charged with rescuing a group of nuns and children, including a newborn infant nicknamed Butch, and transporting them to Pearl Harbor. On their way, the crew sights a Japanese aircraft carrier and attacks, but its torpedoes malfunction, exploding halfway to the target. Attacked and pursued by the carrier's escorting destroyers, Thunderfish manages to escape.
While in Pearl Harbor, the ship's executive officer, Lieutenant Commander Duke E. Gifford, visits Butch at the base hospital, where he encounters his ex-wife, Lt. Mary Stuart, a Navy nurse, and they kiss passionately. Mary is romantically involved with Navy pilot Lt. Bob Perry, Pop's younger brother. Duke pursues Mary anyway but is sent to sea again before anything is settled.
As the sub returns from patrol, the crew spots a Japanese freighter, but again their torpedoes fail to explode. The enemy ship raises the white flag and Thunderfish surfaces and approaches. The freighter is a heavily armed Q-ship that opens fire on the sub. Mortally wounded, Commander Perry orders the sub to crash dive, knowing that he will not survive the maneuver.
Duke, now in command, orders a "battle surface" behind the ship. On surfacing, Gifford orders the boat's deck guns and anti-aircraft guns, as well as numerous portable light and mountable heavy machine guns operated by the deck crew, to fire at will. After the Q-ship's bridge is disabled and the ship is set afire, Duke orders flank speed, ramming the sub into the Japanese ship, holing and sinking the Q-ship.
Thunderfish limps home for repairs to her damaged bow. Back at Pearl 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 that he only did his duty and feels no regret.
Working with the sub base's torpedo specialists, Duke and the crew investigate why the torpedoes are not exploding. When they finally discover the answer, Duke goes to Mary to celebrate, but she rejects him. As he would not allow her into his life when he was at his lowest, she feels that they cannot have a real relationship. Her superior, Commander Steele, overhears the conversation and castigates Mary for ruining her chance for happiness with Duke.
Thunderfish again heads to sea, this time as part of a scouting line searching for a Japanese fleet heading for Leyte to attack the American invasion force. Thunderfish finds the enemy, and although it will reveal their presence, Duke broadcasts the fleet's position. When Pearl Harbor acknowledges the message, Duke salvoes his torpedoes, throwing the attacking Japanese warships into chaos. Despite a battering from Japanese depth charges, Thunderfish sinks 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 downed American flyers while under attack from Japanese fighters. While rescuing the wounded 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. Reconciled, they head to the hospital, intending to adopt Butch.