n the closing days of World War II, Irish crewman Murphy is the sole survivor of the crew of merchant ship Mount Kyle, which had been sunk by a German U-boat and the survivors machine-gunned in the water. Murphy reaches the shore and finds a missionary settlement on the Orinoco in Venezuela, where he is treated by the pacifist Quaker Dr. Hayden.
When Murphy discovers that the U-boat is hiding farther up river under the cover of the jungle, he obsessively plots to sink it by any means, including by using a surviving Grumman J2F Duck floatplane from the Mount Kyle. The floatplane's wounded pilot was shot dead in his hospital bed by the U-boat captain in order to preserve the secret of the sub's location and of the war crimes committed by the shooting of survivors in the water.
Murphy tries to fly the aircraft on the choppy river water and learns how to manipulate the controls by trial and error. He soon finds the U-boat's hiding place and attempts to bomb it using homemade Molotov cocktail bombs, but his effort fails and the mission settlement is destroyed in retaliation. Later, word comes that Germany has surrendered, but Murphy is obsessed with revenge and plans to ram the U-boat with a floating crane owned by Louis, a friendly Frenchman. This attempt also fails when the U-boat dives under him. However, the submerged U-boat is ensnared in a mud bank. Murphy uses the crane to recover an unexploded torpedo fired earlier by the U-boat and drops it on the trapped crew, killing them. Murphy is also killed as the explosion from the torpedo causes the crane jib to pin him to the deck as the floating crane sinks to the river bed.