Near the end of World War II, a British merchant ship is sunk off the coast of Venezuela by a German U-boat commanded by Captain Lauchs. As the sailors fight to stay afloat, the Germans brutally machine-gun each one, watched with horror by the lone, hidden survivor, Irish aviation mechanic Murphy. Days later, the wounded Murphy is rescued at sea by Frenchman Louis Brezan, who pilots a barge on the Orinoco River. Brezan takes Murphy to Dr. Hayden, a refined Quaker missionary who runs a makeshift hospital off the river. Although the crude Murphy insists that his ship was attacked by Germans, Hayden disbelieves that the enemy would bother with their rural area. Under Hayden’s care, Murphy soon recovers, and upon visiting Louis to thank him, the two become fast friends. When Murphy uses Louis’ radio to inform the British Navy about the ambush, the Germans intercept the message. Meanwhile, Lt. Ellis, who was shot while ascending in the merchant ship’s seaplane in an attempt to fire on the U-boat, is discovered and brought to Hayden. Murphy greets him with gusto and readily assents to locate the wrecked plane in the river. As Murphy and Louis sail out to find the plane, Lauchs and his men invade the village and hold Hayden at gunpoint to force her to turn over Ellis. After lying that only one Englishman survived, Hayden listens in horror as Lauchs murmurs the English word “responsibility” and then shoots the injured man. By the time Murphy returns, dragging the plane behind the barge, the Germans have returned to their ship. Upon hearing of Ellis’ murder, Murphy attempts to mask his anger and sadness. He notices Ellis’ flight suit hidden under his hospital bed, and realizing that the lieutenant did not want the Germans to learn of the plane’s existence, sets to work repairing and rebuilding it. He soon has it in the water and admits to a horrified Louis that he plans to use it to attack the U-boat. Louis watches fearfully as Murphy, who does not know how to fly, climbs into the cockpit. The cocky Irishman lifts off unsteadily, then enjoys a shaky but exhilarating ride over the river. Soon after, Hayden sees Murphy flying and, assuming he is going to the mainland, asks him to requisition supplies from the Society of Friends office. Next, Murphy teaches Louis to aid him in creating Molotov cocktails out of gunpowder and petrol bottles, and fashions a release mechanism so he can drop the bombs from the underside of the plane. As he is about to take off, Louis crane-lifts the plane into the water. Hayden arrives to bid Murphy goodbye but, upon spotting the bombs, shouts at him and Louis to stop, stressing that if he fails the Germans may retaliate against the village. Ignoring her, Murphy sets off on his vigilante mission. When the Germans spot the plane, they fire upon it, igniting the bomb fuse so Murphy must struggle to release it before it explodes. Watching the German boat disappear in smoke, Murphy rejoices and returns the village to celebrate. He has broken a rib, and as Hayden wraps it, she vents her frustration with both men by snapping at Louis to “revert to your usual practice of saying and doing nothing.” Later, she apologizes to the hurt Frenchman, kissing him on the cheek. Despite his confidence, Murphy has failed to destroy the U-Boat, and Lauchs soon surfaces and strafes the area to demolish the plane, killing several villagers. Although Lauchs takes Hayden at gunpoint to search for Murphy, he eludes them by clinging to the side of a cliff wall. After the soldiers depart, Murphy apologizes mournfully to Hayden, who counsels him to move forward. However, Murphy cannot let go of his mission, and soon launches a plan to attack the Germans with Louis’ barge. As he motors out onto the lake, a radio announcement reveals that the German Army has been defeated and will soon surrender. Refusing to listen, Murphy heads straight for the U-boat, where Lauchs spots him and wearily shouts through a megaphone for him to turn back, as the war is over. When Murphy continues on, Lauchs orders the men to battle stations and fires a torpedo. By turning sharply, Murphy manages to avoid the torpedo, which slides by and beaches on the nearby shore. Lauchs dives the ship, which submerges too quickly for Murphy to manage to ram into it. He brings the barge ashore, and when Louis sees him loading the torpedo onto the barge crane, he stalks away in frustration, shouting at Murphy, “The world will never build us a monument. The difference is, I know that.” Laboriously, Murphy swings the torpedo back toward the open water, as Lauchs prepares to trap and torpedo him. Murphy floats out over the still water, looking for a sign of the ship. When he sees an air bubble surface, he releases the torpedo straight down. The weapon hits its mark, but as the U-boat collapses, the explosion damages the barge, and a large piece of wood falls onto Murphy, who is trapped as the barge begins to founder. Together, the Germans sink with their ship and Murphy goes down with his.