Captain Tom Wilder, John Wayne, an American Merchant Mariner, is taken prisoner after his ship is seized by the Chinese Communists. After two years in prison, he is helped to escape with both bribery and by disguising him as a Soviet army officer. A large Chinese man transports Wilder to Chiku Shan village without divulging why he was broken out of prison.
The village leader, Mr. Tso, Paul Fix, tells Wilder he has been recruited to transport nearly 200 Chiku Shan residents out of Red China to freedom in the British port of Hong Kong. For the task, Wilder must pilot a stolen, wood-burning, flat-bottomed, 19th Century stern-wheel riverboat. He will need to utilize his detailed memory of the China coast to draw a chart and navigate using an unreliable magnetic compass, and without a chronometer. Finally, he must rely upon the villagers' determination and resources to escape.
The villagers have been planning their escape for more than a year, gradually raising the harbor channel's bottom with stones in order to trap the local Red Chinese patrol boat once it has been lured inside. Sinking sampans loaded with rocks at the channel mouth will cause the patrol boat to run aground while the village escapes. The villagers have also been secretly accumulating arms, ranging from Browning machine guns to Mosin–Nagant rifles and Nagant revolvers. Their plan is complicated by having to bring along the large Communist Feng family so they will not be blamed for allowing the mass escape, including Old Feng Berry Kroeger.
The villagers include the riverboat's Chief Engineer, Tack, Henry Nakamura, a U.S. Navy-trained marine engineer, who helps steal the steamboat. Tack and Wilder bring the stern wheeler to Chiku Shan village where it is loaded with furnace firewood, boiler water, and all their provisions; it is re-christened with the village's name.
Wilder meets and is attracted to a tough and determined American woman named Cathy Grainger, Lauren Bacall, whose father is a medical missionary in the village. Dr. Grainger was recently executed by the Red Chinese following unsuccessful surgery on a political commissar. To prevent her staying behind, Wilder tells Cathy about her father's death just before the villagers leave Chiku Shan, though she refuses to believe him.
Following their plan, the villagers lure the patrol boat into the harbor and trap it there. They flee down the coast in the stolen ferryboat, bluffing their way past a People's Liberation Army Navy destroyer. They disappear into a fog bank, hiding by day in shoreline reed marshes and sailing south only at night. The Fengs attempt to sabotage the escape, first by poisoning the food and water supply, then attempting and failing to take over Chiku Shan during a heavy storm. Cathy eventually comes to terms with her attraction to the gruff Captain Wilder.
A shortage of firewood, drinking water, and boiler water compels the Chiku Shan to pull into the Graveyard of Ships at Honghai Bay. Wilder orders the wrecks stripped of wood and water siphoned from various depressions and abandoned tanks for both the boiler and for drinking water. A loose heavy timber suddenly plows through the stern wheel while mooring the steamboat, snapping two of the paddle blades. Wilder is forced to stay in the Graveyard longer than planned in order to make repairs. At the same time, Cathy goes ashore and returns after learning of her father's fate. The Fengs are put ashore, only to be taken back aboard when the pursuing Red Chinese destroyer begins shelling the Graveyard from a distance. It launches power boats to search for the ferry in the shallow harbor.
Chiku Shan makes a run for it into a marshy estuary, and disappears. Because smoke would reveal their position, the villagers both pole and tow the riverboat through miles of marshlands until reaching the open sea beyond the destroyer's search area. Tack fires up the boiler, and the steamboat proceeds to Hong Kong with the refugees. Their triumphant arrival is hailed by the steam whistles and ship sirens of every vessel in the harbor.
