During the Battle of the Atlantic, convoy HX-25, consisting of 37 Allied ships, is making its way to Liverpool. The convoy’s escort consists of the Fletcher-class destroyer USS Keeling, radio call sign Greyhound, captained by Commander Ernest Krause (Tom Hanks) of the United States Navy; the British Battle-class destroyer HMS James, call sign Harry; the Polish Grom-class destroyer ORP Viktor (with a Royal Navy liaison officer on the radio), call sign Eagle; and the Canadian Flower-class corvette, HMCS Dodge, call sign Dicky.
The escort ships are also under the command of Krause. Despite his seniority, it is his first wartime command.
The convoy enters the “Black Pit”—the Mid-Atlantic gap where they will be out of range of protective air cover. While they are still three days away from the resumption of air cover, high-frequency direction finding (HUFFDUFF) from the convoy flagship reports to Greyhound that it has intercepted German transmissions that are likely from a U-boat (submarine). Greyhound’s crew identifies the surfaced sub heading toward the convoy. Greyhound moves away from the convoy to intercept it based on its bearing and gets the U-boat within firing range, but the heavy seas allow the U-boat to dive before Greyhound can get a visual. After sonar contact is re-established, the submarine tries to slip under Greyhound, but Krause maneuvers Greyhound above the U-boat and fires a full pattern of depth charges, resulting in his first kill.
The crew’s jubilation is cut short as they soon receive reports of distress rockets at the rear of the convoy. A Greek merchant ship was attacked and is sinking. Krause moves Greyhound to assist, evading torpedoes fired by another U-boat in the process. After the rescue Greyhound returns to the convoy receiving multiple messages from the other escorts: a wolfpack of six more subs is staying just out of firing range of the convoy, which Krause suspects is waiting for nightfall in order to attack. As night falls, the attack commences with five merchant ships being torpedoed and sunk. One U-boat torpedoes an oil tanker and escapes Greyhound by using an underwater decoy to waste depth charges. Krause chooses to rescue survivors from the burning oil tanker before proceeding to the action at the rear of the convoy.
The next day, the submariners mount an attack on Greyhound. The captain of the submarine Grey Wolf taunts the convoy and its escorts via radio transmission, threatening to sink them all. Krause learns that Greyhound is down to six depth charges. The U-boats launch multiple torpedo runs, which Greyhound is barely able to evade. Greyhound and Dicky combine to sink one of the U-boats in an exchange of surface broadsides. Dicky receives minor damage due to the close range of the engagement and Greyhound is hit on the port side by one of the U-boat’s deck guns, which kills Krause’s mess attendant George Cleveland and two sailors. During the funeral services, Eagle is attacked and eventually sinks. Krause elects to break radio silence by transmitting a single word, “help” to the Admiralty.
On the last day in the Black Pit, the remaining U-boats mount an all-out attack. One of the torpedoes glances off the side of Greyhound as the destroyer barely manages to evade the other. After heavy fighting, Greyhound sinks the lead U-boat. To everyone’s relief, they spot air support deployed from British RAF Coastal Command. With spotting assistance from Greyhound, a PBY Catalina bomber lines up on the last visible U-boat with depth charges, sinking it instantly. The remaining three subs are assumed to have slipped away.
While assessing damage, Krause receives radio contact from the head of the relief escorts, HMS Diamond, that his relief has arrived and Greyhound is due for repair and refitting in Derry alongside his two surviving companion vessels. The crew receives a “job well done” on their four U-boat kills. As Krause turns over command to a junior officer all present on the bridge gaze at their Captain with new found respect. While setting the new course, passengers and crew of the remaining convoy ships cheer and send up flares to salute Greyhound’s crew for their valor and victory at sea while Krause finally prays and rests.