In Paris, cardsharp and murderer C. J. Dabney strangles American tycoon Williams after Williams sees through Dabney's attempt to fleece him at cards. Although American detective Higgins recognizes the napkin knot around Williams' throat as signature Dabney, he urges French police not to arrest Dabney, so he can catch him in the act. Indeed, as Higgins hoped, Dabney soon finds another luckless victim in Freddie Hunter, an Ohio newspaperman, who was assigned by his boss to escort a troop of high-minded Boy Foresters to France. Freddie intends to "accidentally" miss the boat back to America so he can pursue some fun in Paris without the scouts, whose credo is "Save one man from his own weakness and you can save the world." Once at sea, when the beautiful Duchess Alexandria boards the ship with her father, Grand Duke Maximilian, Freddie changes his mind, and Dabney offers to introduce them. However, Freddie vows to stay away from cigarettes, liquor and women after strict troop-leader Stanley catches him smoking a cigarette. Unknown to anyone, Maximilian and Alexandria are moving to America because they are broke, and hope to regain their fortune through business. True to his word, Dabney arranges an introduction for Freddie, whom he pretends is a millionaire, and Freddie is completely smitten with Alexandria, who gets drunk on champagne and returns his affection. Unaware that Dabney is a cardsharp, Freddie maintains his pose as a millionaire and enters into a running card game with him and Maximilian. Freddie finally puts his foot down with the Boy Foresters after they charge him with breaking his vow. Tommy, the only scout who likes Freddie, convinces him to be honest with Alexandria; however, Freddie learns that she is impoverished as well. When Freddie wins the next card game, he realizes that Dabney engineered his winning hands and confronts him. Freddie is about to become Dabney's next victim when Higgins, who has followed Dabney aboard ship, enters the room. As Dabney has not yet put a napkin around Freddie's neck, Higgins does not reveal his identity, but later warns Freddie about Dabney. Dabney then murders Higgins, steals the diary in which he recorded Dabney's activities, and tells Maximilian and Alexandria that Freddie is a crook. As a result, Alexandria humiliates Freddie in front of the Boy Foresters. Although Freddie escapes by leading everyone to believe he has jumped overboard, Dabney remains unconvinced, and enlists the Boy Foresters to search for him. When Tommy finds Freddie hiding in a cage with Alexandria's dog, he brings Alexandria to see him. Having learned the truth from Freddie, Alexandria searches Dabney's cabin, but is captured by Dabney. Dabney is about to kill her when Freddie, who has climbed onto the anchor to hide from the Boy Foresters, is accidentally lowered past his window. Freddie crashes through the window and narrowly escapes being shot by Dabney, who is then arrested by the ship's officers. When the ship docks, Alexandria and Freddie reunite and break a Boy Forester's rule by embracing.