Captain Gregg, returning from leave, is taken to the Depot ship by launch. The Depot ship loads the submarine 'Tyrant' with provisions, torpedoes etc. The Captain settles in and has a word with his Number One, Martin. Later that evening, he is called to see Captain 'S', who commands the entire flotilla of submarines, and is told that 'Tyrant' and another submarine, 'Strongbow', are to patrol the Norwegian coast with a view to destroying enemy shipping.

In the morning, they set off. Their course is plotted on a map. Various exercises are carried out to ensure that the surface trim is OK. Diving stations. They submerge to 30 feet, then surface. Plot course again. Course is altered to avoid a mined area. The captain continually surveys the sea. They have tea. Some men play cards; one draws. Others play chess. They hear a rumbling noise. Diving stations. Resurface. The Captain is on top while the men get on with chores. He keeps a look-out for enemy aircraft, then goes below for his meal.

The crew are depressed because 'Strongbow' has hit two targets and the 'Tyrant' has been idle up to now. Suddenly, the Captain is called to the bridge as an enemy submarine has been spotted. 'Tyrant' starts firing and eventually sinks the enemy, but only after she has been hit herself. 'Tyrant' is unable to dive until the damage has been repaired. They start working through the night to fix the trouble, as dawn will mean inevitable discovery. Eventually, the repairs are completed, just as enemy planes are heard approaching. The Captain gives the order to dive. The planes drop their bombs as the submarine sinks under the water.

The next day, Martin sees a fishing boat about twenty miles from the coast. There are three men aboard the boat, obviously Norwegians escaping from the Nazis. The submarine surfaces and the Captain asks them to climb aboard. He dives again as soon as they are on. They are made to feel at home by the men. Later, it is reported that there are ships nearby. These turn out to be a floating dock towed by tugs and escorted by destroyers. This is the target 'Tyrant' has been waiting for. The torpedoes are prepared for firing and the submarine carefully creeps nearer.

The Captain gives the order to fire, and they hear an explosion just as torpedo number six is released. The floating dock is destroyed and 'Tyrant' dives to safety at 70 feet. They await the counter attack from the destroyers. Depth charges are released. Suddenly, the boat inexplicably begins to rise and the destroyers move in for the kill. At 30 feet, they regain control and dive to 160 feet, waiting on the sea-bed for the search to end.

The Captain gives orders for all personal papers to be destroyed. He tells the men to lie down so as to conserve air. After eighteen hours, they decide it is safe to rise to periscope depth. However, the boat is stuck in the mud. Measures are taken and the boat rises. The Captain surveys the surface with his periscope: nothing in sight. The submarine surfaces. The Captain gives orders for a signal to be sent saying they have sunk the floating dock.

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