The entire story of "The Demon Lover" is a buildup to the ending, when the titular demon lover, disguised as a taxi driver, apprehends Kathleen Drover and drives away with her. Although it is meant to be a surprise, there are subtle clues throughout the story that direct readers to the inevitability and the possibly supernatural nature of this ending.
The story does not make it clear whether Kathleen's long-lost fiancé is an actual ghost or demon, or whether he has been missing for twenty-five years and has returned. Either way, there is something sinister about the letter she finds in the hallway of her abandoned house that assures her that he will meet her "at the hour arranged."
Her recollection of when she last saw her lost fiancé is vague and sinister as well. He extracts a promise from her to meet him again, and that promise haunts her for a long time, even after she hears that he is "missing, presumed killed" during World War I. The story takes place twenty-five years later, when Kathleen and her family have left London, which is being frequently bombed, for a refuge in the countryside. She has returned to their house to gather a few things but then finds the note, which evokes a feeling of dread. She wonders if her erstwhile fiancé has returned to force her to hold to their past agreement. We understand from her fear that she believes this might be possible.
Kathleen realizes she has to flee and decides to call a taxi, so she can transport the things she has gathered. She is frantic to get to the taxi before the man who wrote the note finds her. She never imagines that the man might impersonate a taxi driver. However, there is a major clue to the ending when on her way out she feels a draft from the basement, as if "a door or window was being opened by someone who chose this moment to leave the house." She walks quickly to the square where the taxi stand is, and she thinks as she enters the taxi that she is finally safe—only to discover that the taxi driver is the man she feared and that he has succeeded in capturing her. She begins to scream and beat on the windows.