During World War I, Captain Ivan Orloff, a Russian combat hero and a notorious playboy, is sent to Bucharest to identify and apprehend a woman spy who has been undermining the Russians' diplomatic efforts in Rumania. On the train to Bucharest, Ivan, whose phony mission has been deliberately revealed to the enemy, is joined by beautiful Countess Diana Dorchy. After a long, flirtatious conversation with Diana, Ivan learns that she is traveling to Bucharest to attend a ball that is being given in his honor. At the ball, Ivan meets Colonel George Gorin, the head of the local Russian secret service, Gorin's fiancée, Baroness Alma Corri, and the talkative Madame Blinis. Ivan then formally introduces himself to Diana and is cornered by Alma and Madame Blinis, all of whom have been labeled as espionage suspects. After a Russian lieutenant is shot by an unidentified killer in the mansion garden, Ivan and Gamble, his valet, return to their rooms just as the killer is sneaking out of them. The next day, Ivan encounters Diana in a Bucharest park and, after determining that a piece of paper that she had been hiding from him on the train is a clothes shopping list, confesses his love. Later, while Ivan prepares for a dinner date with Diana, Alma sneaks into his bedroom and eavesdrops on his conversation with Gamble. Ivan, however, sees Alma reflected in the bedroom mirror and feeds her false information on the whereabouts of a phony diplomatic letter. Alma, convinced that the letter is in a pocket of one of Ivan's suits, connives to search his bedroom, thus satisfying Ivan that she is indeed the spy. While Alma and Ivan, who feigns ignorance of Alma's activities, are flirting in the bedroom, Diana arrives for dinner. Hurt and jealous, Diana tells Ivan that she is leaving Bucharest the next day. Alma, sensing Ivan's love for Diana, instructs Natalie, a cohort who is posing as Diana's maid, to eavesdrop on any conversation between Ivan and the countess. As expected, Ivan rushes to Diana's apartment and, after alluding to his mission, begs her not to leave Bucharest. Although Gamble is shot fatally by the killer in Ivan's rooms, he is able to alert Ivan by telephone before he dies. Natalie then reports to Alma, who tells Gorin of Diana's interest in Ivan's mission and Ivan's love for Diana. To vindicate Diana as well as himself, Ivan tricks Alma into exposing herself as the spy to Gorin. At that moment, however, Natalie and the killer burst into the room and hold Ivan and Gorin at gunpoint. After he is saved by the arrival of the French ambassador and his men, Ivan escorts Alma back to St. Petersburg and is joined on the train by a loving Diana.