The steamship S.S. Mammoth leaves from Havre, France for New York. On board is Princess Olga of Sweden and Gertie, her lady in waiting. Olga, who is really actress Wanda Nash of Brooklyn, has pretended to be a princess to win a movie contract. Due to a mix-up of rooms, Olga meets Joe King Mantell, a famous concertina player, who falls immediately in love with her. Meanwhile, Captain Nicholls receives word that escaped murderer Paul Musko is on board and gathers together five international detectives who are on their way to a convention to help locate him. Musko speaks seven languages and is known to be an actor. As the detectives meet, a mysterious man searches suite B50, belonging to blackmailer Robert M. Darcy, and eavesdrops on the detectives' meeting. Darcy then threatens to expose King's prison record and Wanda's impersonation to Detective Lorel. Although King resists Darcy's threat, Wanda gives him her last £50 and her emerald ring. After Olga dines with King, she discovers Darcy dead in her room. King and his friend, Benton, move Darcy's body to his room and discover that Olga's fifty pounds are missing. When the detectives later search Darcy's room and examine the body, they discover that it was moved from the scene of the crime. They also find Darcy's list of three on-board blackmail targets. The detectives, meanwhile, have deduced that Musko is disguised as passenger Nicholai Petroff, whom no one can locate. Although the detectives believe Musko killed Darcy, when they find a blonde hair on the corpse and King's accordion in Olga's room, they suspect that Olga and King are connected with the murder. King then discovers Benton with Olga's ring, which is engraved "Wanda Nash." After Detective Steindorf finds a stash of passports in room B50, he is murdered. Captain Nicholls then tells Olga she is under suspicion for murder and advises her that he must contact her government. After King promises to announce the killer that night, Wanda confesses her real identity to him. King then receives a murder threat and asks Lorel to protect him. Expecting Benton to cover for him, King waits for the murderer, but Petroff knocks Benton out. Meanwhile, Olga discovers that Lorel has the money she paid Darcy. As King reveals that Musko is actually disguised as Lorel, Petroff arrives and is wounded by Musko. King and Musko then fight until one of the detectives shoots Musko. Petroff reveals himself as a newspaperman who needed a good story and so knocked Benton out so he would not get in the way. In the New York harbor, crowds wait to greet Princess Olga, but Wanda publicly reveals her true identity. Back on board, she swears her love to King, and they kiss.