Behind the Bamboo Curtain of mainland China, after the Communist takeover, Westerners residing in Shanghai are rounded up by the brutal Maj. Ling Wu and his police and interned at the Waldorf House Hotel, pending the identification of a spy known to be sending critical information to the West. During the first evening at the hotel, Sun Lee, an assassin sent by Nationalists, sneaks in to kill an internee and arms profiteer, Ricki Dolmine, for his part in the takeover. Although the assassination is thwarted by two other internees, American doctor Dan Maynard and ex-sailor Knuckles Greer, Sun escapes through a coal chute in the basement. Later, seeing that, unlike the other hotel inhabitants, the expensively dressed, Tangiers-born Rita King is able to come and go at will, the jaded Dan presumes that she has been planted in the hotel to spy on the other internees, especially as she is often accompanied by the new police chief, Col. Zorek. Although he is disdainful of Rita, Dan is also intrigued by her, and one night suggests to her that she is unhappy and like him, has lost "old dreams." She kisses him flirtatiously, but leaves him abruptly, wanting more. On another day, while looking for the alleged spy's radio, Ling instigates a search of all the rooms and begins cruel interrogations of the inhabitants. After warning them about sheltering the spy, Ling brings in Sun and shoots him dead in front of the internees, then announces that their food rations will be cut to a near starvation level. Meanwhile, the street-smart Rita notices Ling leering at young newlywed Leah De Verno, and when Leah's husband Emilio is taken from the building, supposedly for questioning, Rita alerts Zorek, who releases Emilio and arrests Ling as he is trying to rape Leah. Because of his attraction for Rita, Dan is pleased to believe that she may have intervened for the De Vernos, but remains wary of her. Meanwhile, Dan urges the other internees to protect the spy, whoever he is. Later, police come for Dolmine and send him to Manchuria, and also take away Mr. Haljerson, a sickly Norwegian businessman, who dies during interrogation. Troubled by Haljerson's death, Rita talks to Zorek, who offers her a luxurious life in a different hotel as a "protected" woman in exchange for her cooperation, but she tactfully evades answering him. At the hotel, Dan, who is oblivious to Rita's precarious situation, acts jealous. The next day, when young Penny Warren, the daughter of another interned couple, needs hospitalization, Dan asks Rita for help. Although Rita doubts what she can do, two policemen later arrive to escort Penny, her mother and Dan to the hospital, and Dan is able to save the girl from a near fatal intestinal obstruction. Several days later, Carl Hoyt, another internee, discovers that Paul Grant has a radio in his room. Intending to betray Grant to the guards, Hoyt bursts out the hotel's front door, but is immediately shot dead by the guards, who assume he is attempting to escape. Afterward, Dan secretly confronts Grant, who admits that he is with the OSI, but that his radio is damaged and unable to send messages. However, he has received a coded message, alerting him of a waiting submarine, and will try to meet it and then get help for the rest of the internees. After showing Grant the coal chute, Dan assists his escape, but before leaving, Grant gives Dan a coded message of military significance and asks him to try to reach the sub, should he fail. The next day, after Zorek tells the group that Grant has been caught and killed, Dan accuses Rita of alerting Zorek. Hurt by Dan's accusations, Rita counters that had Dan trusted her, she could have helped Grant get away. They argue until Dan realizes that Rita has agreed to become Zorek's mistress in exchange for Penny's lifesaving hospital trip. After apologizing, Dan asks her to help him escape and then asks Knuckles to accompany him. Rita enlists Chinese friends, who set up an explosion to distract the guards, while she drives Dan and Knuckles to the boat of another accomplice. After narrowly escaping patrolmen, Dan and Knuckles cross the Yangzte River, then rendezvous with the submarine. Dan delivers Grant's military information, then reports the imprisonment of the other internees. Meanwhile, unaware of the escape, Zorek reports to superiors in Bei Ping, expecting to be promoted. Instead he is arrested for the embarrassment he has caused, as the United Nations is demanding the release of the internees. Later, as Rita watches the internees depart for their home countries, knowing that she has no protection, Dan shows up, having returned to be with her and share her uncertain fate.