In revolution-torn China, American mercenary O'Hara is entrusted with a perilous mission, to get arms for the helpless authorities in a province ravaged by warlord General Yang. On the train to Shanghai, he meets Judy Perrie, whose father is in league with Yang. Will Judy regret agreeing to lure O'Hara to his doom, and if so, can she make it up to him? The balance of power seesaws to a perilous conclusion.

Chinese war lord General Yang, who controls one province, plans to take over all of China's twelve provinces and control all her silk, rice and opium markets. At present, however, he has only enough weapons to arm his closest men, and many of his soldiers are deserting him. Opposition general Wu uses this opportunity to send American soldier of fortune O'Hara to Shanghai to purchase weapons from American gunrunner Brighton, who will sell to the first bidder. The money, which O'Hara carries in a belt, was collected from Chinese peasants, half a million of whom are being oppressed by Yang. American Peter Perrie and his daughter Judy become involved in the fight when Perrie promises to deliver O'Hara to Yang, then purchase Brighton's weapons himself. Perrie has only six months to live and plans to steal some of the money so he and Judy can return to America, which Judy has never seen. Judy reluctantly lures O'Hara onto a train en route to Shanghai, and they fall silently in love. On the train, Perrie hands O'Hara over to Yang, who promises to follow Perrie by boat and arrive in Shanghai in time to exchange O'Hara for Brighton's weapons. At the Mansion House, a hotel in Shanghai, Wu, who is waiting for O'Hara, suspects the Perries when he finds Judy with O'Hara's monkey, "Sam." Perrie, meanwhile, uses some of O'Hara's money to purchase two tickets for a ship bound for America, which is scheduled to leave the following midnight. Judy, remorseful about her betrayal of O'Hara, tells her father she is in love and warns him not to skip town with the money. The avaricious Perrie is determined, however, and hides the money in the lining of his suitcase. That night, O'Hara escapes from Yang's boat and arrives at the Mansion House wounded. After Wu nurses his wound, O'Hara confronts Judy, who refuses to expose her father, but asks O'Hara to give the bearer of the belt two thousand dollars to escape. When O'Hara uncovers Perrie's hiding place, Perrie shoots him in the hand. O'Hara then shoots Perrie dead, and Judy confesses his identity. Yang and his guards then arrive and take Wu, O'Hara and Judy captive on Yang's boat, demanding the money. All insist they do not know its whereabouts, but after Yang shows them the corpse of one of Wu's agents, Judy offers to reveal the money's location in exchange for O'Hara and Wu's freedom. Yang agrees, but upon sensing O'Hara's love for Judy, cruelly orders him to kiss her goodbye. In their last few moments together, Judy explains her actions on the train, and O'Hara tells her they "could have made wonderful music together." Brighton then wakes up from a drunken stupor and, while searching for some whisky in Perrie's bags, finds the money. When the guards demand the money, Brighton insists that it is American currency and, while trying to escape in the dark, stabs Yang. As the general slowly dies, O'Hara, fearing for the lives of Judy and Wu, tells Yang the world will believe that his guards were disloyal and killed him. Yang, however, gives the guards orders to kill Judy and Wu, while O'Hara pleads with him to let them live so they can tell the story of his greatness. After O'Hara finally convinces Yang to save Judy, Yang orders his men to kill each other for the sake of honor, then dies. As dawn breaks, Judy and O'Hara embrace.