An actress pretending to be a princess is blackmailed and becomes involved with a musician and a murder mystery aboard a ship.
At Le Havre Captain Nicholls (George Barbier) welcomes Princess Olga (Carole Lombard) aboard with her lady in waiting, Gertrude Allwyn (Alison Skipworth). King Mantell (Fred MacMurray) won't give up the royal suite until he sees Olga; then he blames his friend Benton (William Frawley). Mantell offers to fix her bathtub and puts his card with flowers being delivered to Olga; but Gertrude sent them. Captain Nicholls tells five detectives from different countries that an escaped murderer may be on board. Darcy (Porter Hall) tries to blackmail Mantell; but Mantell and Benton throw him out. Benton snatches Olga's purse so that Mantell can return it to her; but he drops it in a fish tank. Darcy tells Olga that he knows she is Wanda Nash, and she gives him money and her ring.
Mantell dances with Olga and tells her she can count on him if she is in trouble. Olga shows Mantell the dead Darcy in her room. Mantell gets Benton to remove the body. Detectives investigate and find the names of Olga, Mantell, and Petroff marked. They question Olga and then Mantell, figuring out that Darcy was in both their cabins, and Mantell left his concertina in Olga's room. Olga goes to get the ring Darcy took, because it has her name on it. Benton shows the ring to Mantell, who reads the name Wanda Nash. Mantell asks Olga why Darcy blackmailed her, and Gertrude makes up a story. Mantell finds the German detective Steindorf (Sig Ruman) wounded and dying. Olga persuades the captain not to let her suffer the indignities of investigation; but he is ordered to investigate everyone.
Mantell promises to reveal the murderer. He calls Olga Wanda and gives her the ring. She says that a title got her a movie contract, and he hopes for publicity with a princess. Mantell gets a note that he is next. At a show he plays his concertina and sings. Olga asks the French detective Lorel (Douglass Dumbrille) to be with Mantell. Benton is grabbed and knocked out. Olga collects money and finds a banknote of Darcy's that questioning reveals came from a poker game with Lorel and others. Mantell tells Lorel that the escaped murderer did these murders too, and he suspects him. Lorel pulls a gun, but a man intervenes and is shot. Mantell fights Lorel, who is shot by the man. Olga is welcomed in New York as a princess; but she says she is from Brooklyn and runs off. Mantell hears of this; he finds her, and they kiss.
This mystery is combined with romantic comedy, satirizing the absurd privileges of royalty and what actors will do to get into movies.