Young Deck Officer Arthur Hawkins (Thomas Brodie-Sangster) leaves his pregnant wife Maud (Nathalie Emmanuel) to join the San Demetrio. Shipping Manager Robert Dodds (Brian Cox) promises to keep an eye on Maud. Loading a vital and highly volatile cargo of Texas aviation fuel for the Royal Air Force, they set off in a convoy of other ships for England. On board is an American, Oswald Preston (Blair Redford). In the High Court in London, a court case starts.

The convoy is attacked by a German Battleship. In a suicidal act of incredible courage, the outgunned HMS Jervis Bay engages her to give the convoy time to escape. But watched in horror by our crew, the battleship sinks the Jervis Bay and targets the convoy. Several cargo ships are sunk and then the San Demetrio herself comes under fire.

With men killed, on fire, badly damaged, the captain orders the crew to abandon ship into the lifeboats. The next day the other lifeboats are rescued, but one, with sixteen men aboard, several wounded, is missed. They endure a terrible storm and freezing cold. At last they see a ship on the horizon.

The ship is their own “San Demetrio”, horribly damaged, still on fire and liable to blow up at any time. As Chief Engineer Pollard (John Malkovich) puts it: their choice is stark. Stay on the lifeboat and freeze to death or climb back on board and get blown up. In London, Dodds breaks the news to Maud of the attack on the convoy. After another night in the lifeboat and close to death, Hawkins has no choice but to board the burning ship.

A U-boat submarine starts to hunt our ship. Over enormous odds, the crew manage to fight the fires, but the injured crew men are weakening, especially Boyle. Back in London, Maud breaks the news to Hawkins’ parents and furiously defends his work in the merchant marine. The court case continues.

Pollard works to get the ship moving, but the bridge and all the navigating equipment have been destroyed. The U-boat finds the San Demetrio and prepares to torpedo her unbeknownst to the crew who desperately attempt to get the engines working.

In London Maud reads German claims that the whole convoy has been sunk and she has to hide her grief at a wedding . On board the San Demetrio, the injured Boyle falls unconscious, and they discover the serious injuries he has kept hidden.

Preston reveals he wants to get to England to join the RAF. The valiant Boyle succumbs to his injuries and Hawkins blames himself. In London, Dodds announces that two of the lifeboats have been rescued, but he must admit that Hawkin’s boat was not found, and they are presumed dead. Maud has to tell Hawkins’ parents the news.

On the ship, they fight a full-blown hurricane. They survive and sight land at long last, but don’t know where they are. Hawkins warns that if it’s German occupied France or Norway, he will sink the ship. In London, Winston Churchill is informed about the San Demetrio and orders the ship protected and brought home safely.

Maud and Dodds go to Glasgow to meet the ship. A crowd is shocked at the terrible damage. Hawkins is weak but greets Maud passionately. The crew, some on stretchers, are escorted ashore through a Royal Navy honor guard. As Hawkins goes, in a moment of huge respect, the guard snap to attention and sound “Captain Going Ashore”.

The Judge delivers his findings in the British High Court in London, giving each man a salvage award. Hawkins, happily drunk, announces the birth of his and Maud’s daughter. After attempting to track Oswald Preston down to give him the ship’s ensign per the crew’s wishes, Dodds informs Hawkins that Preston has disappeared without a trace.