In 1926 China, during its Warlord Era, Navy Machinist's Mate First Class Jake Holman Steve McQueen transfers to the Yangtze River Patrol gunboat USS San Pablo as their new engineer. Almost immediately, he upends the crew by refusing to use the local coolies for most of the duties and instead takes personal charge of the engine room. He quickly forms a wary bond with a seasoned sailor named Frenchy Richard Attenborough, whose easygoing bravado masks a tougher, gentler side.
When an engine malfunction props up a problem Holman had already spotted but the captain refused to repair, the chief Chinese engine-room worker dies. Lop-eye Shing, leader of all the Chinese workers aboard, blames Holman, and the captain orders a replacement to be found. Holman selects Po-han [MAKO], a simple, genial man who can keep the vital machinery running, even as the cultural clash on board begins to heat up.
On shore leave, the crew visits a bar and brothel run by a former sailor. A high-priced virgin named Maily [Emmanuelle Arsan] captures their attention, and Frenchy discovers that her freedom comes at a price she must pay. The tension on the ship grows when Holman’s nemesis Stawski [Simon Oakland] teams up with Shing to push Po-han off the vessel. In response, Holman proposes a boxing match to settle the dispute; if Stawski wins, he earns money for Maily, but if Holman wins, Po-han keeps his job. The clash is interrupted by an alarm, and the crew rushes back to the San Pablo, just as a Chinese mob roars with anger over a recent clash between British warships and Chinese warlords on the Yangtze.
With the onset of the Chinese Civil War, the San Pablo is ordered to stay neutral while continuing to rescue Americans upriver. Holman is ordered to help retrieve Jameson, a missionary, and Shirley Eckert, his schoolteacher assistant [Candice Bergen], when Po-han is sent ashore by Shing amid the chaos. After a mob attack, Holman defies orders and shoots Po-han to end his brutal death on shore, a decision that further inflames tensions back at port.
Port life becomes openly hostile toward the sailors. Jameson and Eckert set out toward a new mission, while Frenchy attempts to buy Maily’s freedom; instead, she is put up for auction. Holman helps Frenchy raise funds to outbid her captors, but a brawl breaks out and the trio barely escapes. In a quiet, almost ceremonial moment, Frenchy and Maily wed unofficially in an empty church, sealing a fragile, unofficial bond formed in the crucible of war.
Holman and Eckert begin to grow close, even as he worries about his leaving China and the fate of their fragile romance. As winter arrives, the San Pablo becomes trapped by dropping river levels, and shore leave is halted as Chinese workers abandon the crew and a flotilla of locals blocks the ship. To see Maily, Frenchy swims through the freezing water, but pneumonia takes him, and Maily is killed by antiforeign forces who then frame Holman for her murder. The ship is surrounded, and the crew pressures Holman to surrender; the captain fires a Lewis gun, ending the immediate threat.
Captain Collins [Richard Crenna] decides to rescue Jameson and Eckert from their mission amid anti-foreigner riots in Nanking. The San Pablo confronts a line of tethered junks, sparking a fierce engagement. Holman slices through the ropes, killing a Chinese militiaman—only to realize the young man was one of Jameson’s and Shirley’s students. The gunboat presses on toward the mission, where Jameson and Eckert refuse to abandon the work they began.
As the mission comes under attack, Jameson is killed and the patrol pulls back. Collins is killed in the fighting, and Holman assumes command of the ship under fire. Shirley and Holman reaffirm their love; he promises to follow her to safety, even as he fights to enable their escape. He blows through enemy lines, covering the retreat, and falls fatally wounded just as he nears the others. In his last moment, he whispers a bewildered, haunting question:
> I was Home ... what happened? What the hell happened?
With Holman gone, Shirley and the remaining sailors reach the San Pablo, which slowly but steadily steams away, carrying the memory of a man who chose duty and loyalty over ease, even as the river carries the wreckage of a world in turmoil.
