Scotland Yard Inspector Sanderson goes to the offices of attorney Cedric Mason to request his aid on a missing heir case. While they are talking, Ellen Curtiss arrives to claim the Cosgrove estate and is informed by Mason that another woman claiming to be Ellen Curtiss has already filed title to the inheritance left by her late aunt. Later, at the Red Boar Inn, Sanderson meets the second Ellen Curtiss. Observing their meeting is amateur sleuth Bulldog Drummond and his pal Algy Longworth. His curiosity aroused, Drummond visits Sanderson's home, accompanied by Algy and cub reporter Seymour, and finds Sanderson dead, the victim of a bullet. Hiding in the closet of the apartment is the first Ellen, who visited Mason's office earlier, and who tells Drummond that she is innocent and that the inspector was shot through a window. While Drummond waits for the arrival of Inspector McIver, Algy and Seymour take Ellen to Drummond's apartment. Later, at the apartment, Ellen convinces Drummond that the other girl is an impostor, and Drummond arranges for the the two Ellens to meet at the Red Boar. There, the two Ellens confront each other, and after the second Ellen demonstrates an all-inclusive knowledge of the Cosgrove family history, the first Ellen convinces Drummond that her rival must have been briefed by someone. Drummond then visits Mason's office, where he meets William Cosgrove, brother of the deceased, who has only been willed one shilling by his wealthy sister. When Cosgrove identifies the second Ellen as his niece, the first Ellen, claiming that there is a plot against her, insists that her governess, whom she remembers only as "Nanny," can bear witness to her identity. To locate Nanny, Drummond searches the Cosgrove mansion and finds a book containing her address. After Nanny is found and identifies the first Ellen as the true heiress, Drummond, suspicious, tricks her into admitting that she had been coerced into lying and that the second Ellen is the actual heiress. Drummond then proves that Mason was in league with the phony Ellen to collect the estate, and that he murdered Sanderson after the inspector uncovered his plot.