When detective Hugh "Bulldog" Drummond flies into London for the birth of his ex-partner Algy Langworth's first child, Inspector Nielson of Scotland Yard asks Drummond to meet him at Rockingham Lodge. En route to the lodge, a woman feigns being hit by Drummond's car and then drives away while Drummond runs to help a wounded man whose body sinks into the nearby marsh. Drummond discovers she is Phyllis Clavering, who resides next door at Greystone Manor, where she is being held by her guardian, Norman Merridew. After receiving a note from Phyllis regarding an envelope she left in his car, Drummond reads its contents to Nielson and Merridew, but it contains only jumbled nursery rhymes. While Merridew maintains Phyllis is delirious following the accidental death of her brother Ted, Phyllis tells Drummond the envelope contained proof that Merridew murdered Ted and that the man killed at the marsh was her chauffeur, who was trying to help her escape. Nielson, who is on vacation, is unconvinced of Merridew's criminality and therefore asks Drummond to drop the case and leave England immediately. Undaunted, Drummond procures the bullet that killed Ted and matches it with a bullet Merridew shoots into a dummy, which he thinks is Drummond. While Algy and Drummond's butler Tenny abduct Merridew's sister Natalie, thinking that she is Phyllis, Drummond finds Phyllis and they hide in a chest. Merridew finds Drummond, however, and leads him to a secret room where he has been manufacturing shrieks to convince Phyllis that she is crazy. Merridew then confesses to counterfeiting Phyllis' war bonds in order to steal her inheritance. After a struggle with a gun delivered by Phyllis, Algy arrives and wounds Merridew. Nielson then confesses he knew Merridew was guilty all along, telling Drummond he has been useful in his "own quiet, acrobatic way." Algy learns he is the father of a boy and Drummond and Phyllis drive off into the fog to make their wedding arrangements.