In February 1942, amid the intense Battle of the Atlantic, convoy HX-25 sets sail towards Liverpool, comprising 37 Allied ships. Leading the charge is the USS Keeling DD-548, known by its radio call sign "Greyhound", under the command of Commander Ernest Krause (Tom Hanks), a member of the United States Navy. He commands alongside several escorts including the British HMS James (co-captained by Dominic Keating), the Polish ORP Viktor (commanded by Maximilian Osinski), and the Canadian HMCS Dodge K136.
Despite his extensive naval training, this marks Krause’s inaugural command during wartime. Just two months prior, he was romantically involved with Evelyn Frechette (Elisabeth Shue) in New York City, exchanging heartfelt Christmas gifts while discussing their future together.
Krause’s crew features a dedicated team including Lieutenant Commander Charlie Cole (Stephen Graham), his executive officer, and George Cleveland (Rob Morgan), serving as Mess Attendant 2nd Class. Among others are Lieutenant Watson (Tom Brittney), sonar man Red Eppstein (Karl Glusman), and fire controller Melvin Lopez (Manuel Garcia-Rulfo).
As the convoy ventures into the perilous "Black Pit," a Mid-Atlantic zone devoid of air support, the atmosphere thickens with tension; they are nearly 50 hours from safety. High-frequency direction finding intercepts alarming German transmissions that suggest the lurking threat of U-boats. The tension escalates as Greyhound spots an aggressor submarine approaching. Displaying quick wit and decisive action, Krause maneuvers his ship expertly, utilizing sonar to pinpoint the U-boat’s location before effectively sinking it with depth charges.
However, the relief of their victory is short-lived. Distress signals beckon from the rear of the convoy as a Greek merchant ship succumbs to another U-boat attack. With tactical dexterity, Krause steers Greyhound into action, skillfully evading torpedoes while rescuing the beleaguered sailors before returning to the convoy amidst rising peril. Reports indicate a wolf pack of six U-boats advancing—Krause deduces they are poised to strike once darkness falls, with five positioned at the front and one lurking at the rear, all while they await air support to arrive.
An oil tanker is soon struck by a U-boat torpedo, leading to a critical choice for Krause. He decides to prioritize rescuing those in peril rather than pursuing the other aggressors. The naval conflict intensifies as the U-boats launch their assault by targeting five merchant vessels that succumb to enemy fire. The following day, with mounting casualties and dwindling supplies—including just six depth charges left—Krause faces the menacing taunts of the U-boat captain while navigating the increasingly treacherous waters.
As night descends, the ship's crew musters all their courage in the face of adversity. Tragically, during a fierce confrontation, they suffer losses, including Cleveland, who is killed alongside fellow sailors when their ship is struck. The calamity of war weighs upon Krause as he grapples with the devastating impact on morale.
With 14 hours remaining until air support arrives, Krause breaks radio silence in desperation, sending a simple plea—"help"—to the Admiralty which, in turn, relays a new rendezvous point just a few hours away. Tensions escalate as the remaining U-boats intensify their assault in a final bid to demolish the convoy. In a fierce exchange of fire, Greyhound manages to take down the lead U-boat, marking a pivotal moment as air support from the British RAF Coastal Command arrives just in time, allowing for the remaining U-boats to disperse before they can be engaged.
After surviving the predicaments of battle, Krause receives communication from HMS Diamond confirming his much-needed relief. As Greyhound prepares for repairs in Derry, the crew reflects on their accomplishments—the sinking of four U-boats—and are met with cheers from their grateful companions.
As the surviving team sets course towards safety, Commander Krause finds a moment of respite, retreating to his cabin to absorb the events of their harrowing journey, thankful yet solemn, after experiencing the weight of leadership in wartime.
