On patrol in the South Atlantic, the American Buckley-class destroyer escort USS Haynes is led by Captain Murrell [Robert Mitchum], a former merchant mariner who has joined the Naval Reserve and is still recovering from injuries after his previous ship was sunk by a German U-boat, leaving him adrift for 21 days. The crew watches him navigate doubt and determination as he assumes command, trying to prove his capability under pressure.
A nearby German U-boat turns the tension into a tense game of cat-and-mouse. The submarine is commanded by Kapitän zur See von Stolberg [Curd Jürgens], a wily World War I veteran who loathes fighting for a Nazi regime he cannot respect. The two captains anticipate each other’s moves, each trying to outguess the other in a high-stakes chess match on the seas.
Murrell subjects von Stolberg’s crew to hourly depth-charge attacks, hoping to force the U-boat to surface where its vulnerabilities are greater. Von Stolberg studies Murrell’s pattern, calculating the rhythm of the American ship’s attacks. He succeeds in torpedoing the Haynes, sending a shock of fear and urgency through the deck. In a calculated ploy to make the damage appear worse than it is, Murrell orders fires lit on the deck, prompting a rapid, controlled abandonment of ship while he keeps a skeletal crew manning the bridge, engine room, and one of the three-inch guns.
When von Stolberg surfaces to finish the Haynes off, Murrell makes a bold, desperate move: he rams the U-boat with Haynes’ bow, riding up over its foredeck and becoming entangled. With his own vessel foundering, von Stolberg orders his crew to set scuttling charges and abandon ship, sealing the danger for everyone aboard the Haynes.
Murrell, the last man aboard, spots von Stolberg on the conning tower and refuses to abandon his mortally wounded executive officer Heinie Schwaffer [Theodore Bikel], who has stood by him since their academy days. Murrell launches a line to the submarine and pulls the pair free, a risky rescue that defies the sinking odds. Lieutenant Ware [David Hedison] returns in the captain’s gig with a mixed party of American and German sailors, racing up cargo nets to pull the final survivors to safety amid the looming explosion of both vessels.
Rescue comes after another U.S. Navy ship arrives, and the German seamen honor Heinie Schwaffer with a burial at sea while Murrell and his crew pay their respects on the Haynes’ stern. In a final, quiet gesture of uneasy friendship, Murrell offers von Stolberg a cigarette as a fragile olive branch. The two captains exchange loaded words that acknowledge their shared losses: von Stolberg confesses, “I should have died a dozen times over, Captain. This time, it was your fault.” Murrell replies, “Fine, next time I won’t throw you the rope.” Von Stolberg answers with a wry smile, “Oh, I think you will.”