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 DD-548, radio call sign "Greyhound", captained by Commander Ernest Krause of the United States Navy; the British Tribal-class destroyer HMS James F80, call sign "Harry"; the Polish Grom-class destroyer ORP Viktor H34, call sign "Eagle"; and the Canadian Flower-class corvette, HMCS Dodge K136,call sign "Dicky". Krause is overall commander of the escort ships, but despite his seniority and extensive naval education, 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. High-frequency direction finding from the convoy flagship intercepts several German transmissions, indicating the presence of U-boats. Greyhound identifies a surfaced sub heading towards the convoy and moves to attack. The submarine tries to slip under Greyhound, but Krause maneuvers his ship above the U-boat and sinks it with a full pattern of depth charges.

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 by another U-boat and is quickly sinking. Krause moves Greyhound to assist, and with careful maneuvering evades torpedoes fired at his destroyer. The surviving Greek sailors are rescued, and Greyhound returns to the convoy just as the bridge receives multiple messages from the other escorts: a wolfpack consisting of six U-boats is staying just out of firing range of the convoy; Krause suspects they are waiting for nightfall, when the escorts will have no visibility. The attack commences that evening with five merchant ships being torpedoed and sunk. One U-boat torpedoes an oil tanker and escapes Greyhound by using an underwater decoy device, tricking the crew into wasting most of their remaining depth charges. Krause chooses to rescue survivors from the burning oil tanker first rather than going to the aid of the other ships. This decision results in the loss of a cargo ship.

The next day, the wolfpack targets Greyhound. A member of a U-boat's crew, identifying himself as Grey Wolf, taunts the convoy and its escorts via radio transmission. Krause learns that Greyhound is down to just 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 combat 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 of his sailors. During the funeral service, Eagle is attacked and eventually sinks. Krause, aware that doing so might expose the state of the escort fleet, elects to break radio silence by transmitting a single word, "help", to the Admiralty.

With the convoy close to reaching air cover, the remaining U-boats mount an all-out assault on the destroyers. After heavy fighting, Greyhound sinks the lead U-boat with a full broadside. Air support deployed from British RAF Coastal Command arrives and Greyhound fires, marking the last visible U-boat location, allowing a PBY Catalina to depth-charge and sink the sub. The two surviving wolfpack subs quickly flee.