Igor Sargoza shoots down Peter Roma's biplane over an orphanage in China where Irene Roma teaches. Wounded, Peter entrusts Irene with a broken amulet that he must deliver to San Francisco; the other half is waiting there, and together the two halves will unlock $5 million to be used for the purchase of munition for China to defend itself during the Second Sino-Japanese War. With Sargoza close on her trail, Irene misses the last ship evacuating foreigners and seeks help from Howard Barclay, who her brother told her would lend a hand. Barclay's friend Johnny McGinty, an Irish-American photojournalist also hoping to leave Shanghai the next day, is asked to assist Irene.
Because Irene lacks an American passport, Barclay suggests that she and Johnny marry so she can be added to Johnny's passport, with the plan to annul the marriage once they reach San Francisco. Johnny agrees and steps into his adjoining room to pack, but is accosted by a gun-wielding Chinese man who escorts him to Sargoza's car. At the border of the international zone, Japanese officers led by Fuji Yokohama stop the car and release him.
Johnny tracks down Barclay and Irene at the home of Wu Chang, a minister who performs the marriage ceremony. Back at the hotel, a wedding present—an ornate Mandarin incense burner—arrives from Lun Sat Li, a friend of Johnny's. Unbeknownst to them, the courier was waylaid and an explosive device planted. Barclay suggests hiding the amulet inside the incense burner. As bombing rains down on the city, the three flee to Lun Sat Li's home. Sargoza and his men force their way in, and the trio escapes in a power launch through a floor trap door. They are granted passage on the ship that will evacuate American citizens come morning.
In their stateroom, they find Yokohama waiting for them, demanding the amulet. He grabs the box from Irene and prepares to open it when Sargoza appears. Sargoza takes the incense burner from Yokohama and locks the four in a room while he lights a match to melt the candle and retrieve the amulet. The incense burner explodes, killing Sargoza. Yokohama then reveals that he was the mastermind behind intercepting the gift and planting the bomb. He also tells Johnny that Barclay is a Chinese agent empowered to purchase munitions with the funds waiting in San Francisco. However, with President Roosevelt’s embargo on government shipments to nations at war, those funds cannot be used for that purpose.
Barclay says goodbye to Johnny and Irene and slips the amulet—which he never placed in the burner—into Irene's hand. With the danger behind them, Johnny and Irene plan to sail for San Francisco and, this time, stay married for real, stepping into a precarious future built on trust, risk, and the fragile hope of peace amid war.
