A bank robber, Dakin Barrolles (Edmund Lowe), has escaped by jumping into the Thames and swimming away from police boats looking for him. He climbs aboard a yacht and finds Xandra (Joan Bennett) and John Lasher (Edmund Lowe) honeymooning. Dakin threatens them with a gun that he never shows, which is a definite tell. He forces Xandra to bandage his ankle, all the while humiliating John Lasher who seems much too drunk to mind much. Before he dashes away Dakin takes Xandra's locket with a picture of both herself and her husband as a good luck charm.
Dakin enlists in the army, which with WWI on, means he will immediately depart for France far from the police. When Dakin is badly injured and his face blown off, the plastic surgeon has only the locket with John Lasher's picture to go by, and Dakin wakes up looking like Sir John. Meanwhile a captain has come looking for Dakin who is of course still wanted, and instead finds who he thinks is Sir John, who is also still missing. His reluctant wife who has become a party girl in her husband's absence comes to retrieve him back to England and his estate. Dakin does not originally plan to take advantage of Xandra or the situation, but complications ensue.
The plot is outlandish, but I think that is part of what makes it so much fun. It is much more than some random melodrama, with a titular Scotland Yard detective who apparently has so much time on his hands that he can devote all of that time and all of his men to trying to catch Dakin Barrolles in the act, when he doesn't even know for sure that this IS Barrolles.
A fun and realistic touch? The nurse in France assisting the plastic surgeon speaks in French to the English who are too polite to correct her until she catches herself and segues into English. She does this several times. Also fun is seeing Donald Crisp playing a villain, and not a very bright one at that.