The story begins with a young man, John Ordley, visiting Kirby to seek help regarding his fiancée, Miss Joan Richards. John is concerned that Joan has fallen under the influence of Ahmed Bey, a mysterious and charismatic figure who recently arrived in their town. John describes Ahmed Bey as a wealthy, well-educated man claiming to be an Arabian nobleman, but with a suspiciously mixed-race appearance and unsettling yellow eyes. John believes that Ahmed Bey has cast a spell over Joan, making her act unnaturally submissive and distant.

Kirby, an expert in occult matters, agrees to help and accompanies John to his hometown. They arrive at John's old-fashioned mansion, situated near the edge of a swamp. The neighboring house belongs to the Richards family, who are hosting a party that evening. John invites Kirby and his companion to the party, where they meet Joan and Ahmed Bey. Kirby quickly deduces that Ahmed Bey is not an Arab but a Haitian mulatto named Loup, a fugitive wanted for murder and rumored to be a voodoo priest.

Kirby's suspicions are confirmed when he observes Loup hypnotizing Joan on the veranda. Loup commands Joan to steal money from her father's safe and prepare to elope with him. John confronts Loup, but is easily overpowered by Loup's jiu-jitsu skills. Kirby then intervenes, revealing his knowledge of Loup's true identity and threatening to expose his secrets unless he releases Joan from his spell. Fearing Kirby's threat, Loup reluctantly frees Joan, who regains her senses and rushes to John's side.

As Loup slinks away, defeated, a gunshot suddenly rings out, and Kirby is struck by a bullet. He quickly recovers, realizing that Loup had found John's pistol and attempted a final act of revenge. Kirby's quick thinking and bravery ensure the safety of Joan and John, but the story ends abruptly with Kirby about to pursue the fleeing Loup.