Adapted from the novel by Nicholas Monsarrat, a captain during World War II tries to destroy U-boats as they accompany convoys in the Atlantic.
In 1939 the merchant captain Ericson (Jack Hawkins) is given a navy ship with an inexperienced crew. The First Lieutenant is strict and hated. They put out to sea and get three weeks training. The lawyer Morell (Denholm Elliott) joins the ship. They go to war with a convoy. The sea is rough.
The First Lieutenant comes back from leave drunk, and other officers plot against him. At the captain’s table the First Lieutenant has stomach pain, and the others laugh. Ericson says he was left behind because of an ulcer. A ship is sunk, and Ericson orders men to go in a boat to pick up the survivors.
After a year they come home for a refit. Tallow (Bruce Seton) is taken care of by his sister. In 1941 they learn more U-boats are in the area. Ericson stops to pick up survivors. With a U-boat nearby he passes through men in the water. When they are killed, he is called a murderer. Ericson feels bad and drinks until he passes out.
Ericson has to stop engines for a repair, and he orders quiet. After the repair they catch up with the convoy. They detect something small. Ericson sees a U-boat on the surface ahead, and they shell it. After it submerges, they drop depth charges. They see oil and remains come to the surface and cheer.
In port they go to a party. Lockhart (Donald Sinden) escorts Julie (Virginia McKenna) and makes a date with her. Tallow sees that his house was destroyed and learns that his sister was killed. Morell visits his wife who is an actress busy with her career. He gets a call from her friend and realizes he lost her. Lockhart kisses Julie, who says she is being transferred. He says he might marry her but not in wartime.
At night the ship is hit by a torpedo. The ship is sinking, and Ericson orders men to abandon ship. They jump into the water and swim to the raft. The ship goes down. A lifeboat is overcrowded, and men in the water pray. Julie hears the report about the ship. Two small lifeboats are rescued.
Ericson tells Lockhart that Morell’s wife took it well. Ericson tells Lockhart he is promoted and can have his own command, but Lockhart says he wants to stay with Ericson. On his new ship Ericson remembers the trauma. Lockhart goes on a picnic with Julie and says he wants to finish the war only with Ericson. He expresses his grief, and she warns him to be careful.
In the North Sea better weather means more danger. They attack a submarine and see oil, but Ericson thinks it did not sink. Ericson tells Lockhart that the war has changed to simply killing the enemy. He continues the search and asks the doctor to help him stay awake and takes a pill. A U-boat surfaces, and Ericson orders men to fire on it. Men climb aboard before it sinks. In 1945 a U-boat surrenders.
This realistic war drama portrays the boredom and cruelty of war as men leave their worthwhile professions to dedicate themselves to killing enemies in difficult circumstances.