In 1944, toward the end of World War II, Commander Casey Abbott of the United States Navy is assigned the strategic mission to retrieve an underwater mine developed by the Japanese, which is undetectable by radar. Casey and the crew of his submarine, The Starfish , transverse a Japanese shipping route to the edge of the minefield, where Navy diver Wes Barton leads an expedition in search of the mines. Soon after Barton submerges, however, the sub’s radar reveals an enemy destroyer approaching, thus forcing Casey to abandon his mission and submerge his sub, a tactic that results in Barton’s death. Casey’s executive officer Lt. Commander Don Landon protests Casey’s decision, implying that he may have been motivated by the fact that Barton had been dating Casey’s former sweetheart, Navy nurse Helen Blair. Freddy Warren, a young sailor, is also traumatized by Casey’s decision. Back at their base in Guam, Casey is vindicated by Vice-Admiral Charles Lockwood, but Lockwood rejects Casey’s suggestion to follow a Japanese fighter through the mine field and chart its course as it threads its way through the explosives. Helen, relieved to see Casey, confides that she never loved Barton and reassures him that he was not influenced by personal considerations. Assigned a new mission to test a circuit developed by the lab to detect the mines, The Starfish suffers an enemy attack in which its sonar and radar are knocked out and Freddy is gravely injured. To save Freddy’s life, Casey orders the sub to surface and return to port, thus aborting his mission. After safely reaching Guam, Casey and Landon are assigned to destroy a Japanese island being used as an enemy base. Come nightfall, Landon leads the raid, but when their explosives detonate ahead of schedule, the men find themselves adrift in a sea of fire. After all but one man is rescued, the enemy ships retaliate, sending The Starfish plunging down into the sea. His sonar and radar disabled by attack, Casey has no choice but to follow a Japanese freighter through the treacherous strait, making a chart of the deadly mine field as the sub limps along. Once outside enemy territory, The Starfish resurfaces, but the vessel has been irreparably damaged, forcing Casey to give the order to abandon ship. Although the raft carrying the survivors is rescued, Casey blames himself for the loss of sixty of his men. Once ashore, Landon discovers that he has been denied the post of commander due to a negative performance review written by Casey. When Landon challenges the review, Casey states that Landon is incapable of making painful decisions during emergencies, rendering him unfit for command. Freddy, recovering in the hospital, apologizes to Casey for questioning his decision about Barton. At the inquiry into the loss of The Starfish , Casey is vindicated and given the command of The Sea Ray with Landon as serving as his executive officer. The Sea Ray , and the Navy subs in the area, designated “The Hellcats,” are then assigned to penetrate the Tsushima Strait, a critical trade route for the Japanese, and launch an attack that will severely cripple the lifeline that provides food and supplies to the islands of Japan. If sonar fails, the men are instructed to use the chart mapped earlier by Casey. Upon reaching the strait, The Sea Ray ’s sonar is knocked out, but the sub successfully navigates the mine field using Casey’s chart. On June 9th, the attack is launched, destroying a total of twenty-two ships and one submarine. When The Sea Ray ’s propeller blade becomes jammed, the sub is forced to surface for repair, and Casey dons a diving suit and dives below to fix it, leaving Landon in command. Just as Casey becomes entangled in a cable, a Japanese destroyer is spotted on the horizon, and Casey radios orders for the sub to submerge. When Landon balks, Casey reminds him that the crew is now his responsibility, prompting Landon to cut Casey’s cable and submerge, casting Casey adrift. After torpedoing the destroyer, Landon peers through his telescope and, spotting Casey bobbing on the surface, immediately resurfaces and saves him. With their mission completed, Landon apologizes to Casey for misjudging him. As the sub sails into port, Helen waves to Casey, who then turns to Landon and asks him to be his best man at their wedding.