Lieutenant Taylor (John Mills) and the rest of the crew of the submarine Sea Tiger are given a week’s leave after an unsuccessful patrol. Hobson (Eric Portman) goes home to save his marriage, while a reluctant Corrigan (Niall MacGinnis) heads off to his wedding. Then the crew are called back to duty, much to Corrigan’s relief, though he later has second thoughts. Sea Tiger is assigned the top secret mission of sinking Nazi Germany’s new battleship, the Brandenburg, before she enters the Kiel Canal to begin sea trials in the Baltic Sea.
On their way, the submarine picks up three shot-down Luftwaffe pilots from a rescue buoy. When the submarine enters a minefield, an airman panics and reveals that the Brandenburg is further ahead than believed. Taylor decides to take a desperate gamble and enter the German-controlled Baltic in pursuit.
When the Brandenburg is spotted, Sea Tiger fires all its torpedoes, then dives to evade German destroyers dropping depth charges. By expelling oil and other debris, Taylor fools the Germans into believing that the submarine has sunk. They leave, but Sea Tiger no longer has enough oil to return to Britain.
Taylor decides to have his crew abandon ship near a Danish island. Hobson, a former merchant seaman who speaks German and knows the port on the island, persuades Taylor to let him go ashore in one of the airmen’s uniforms to find oil. He succeeds. Sea Tiger refuels while Hobson and other crewmen hold off the German garrison. While they hold them off, Pincher (the cook) is killed and Oxford and Lieutenant Johnson are wounded, but they all get back to the sub.
When they return to base, the crew hear they sank the Brandenburg. Waiting for them are Corrigan’s fiancée and Hobson’s wife and son.