This story is about a murderer who wrestles with his conscience.
The main character, a young man called Markheim, goes to buy a gift from an antique dealer’s shop, and murders the dealer to steal his money. Left alone in the shop, he is soon disturbed by the arrival of a mysterious visitor who seems to know all about him. They fall into a philosophical discussion about his crime and what is to be done about it. Markheim’s misery of mind is made clear during this exchange, but he is also puzzled by his visitor, who never identifies himself clearly.
Finally, Markheim decides to give himself up to the authorities. Hearing the front doorbell ring, he goes down to answer it. The maidservant who works at the shop, has returned. The crux of the whole story lies in the final sentence, when Markheim addresses the maid:
‘You had better go for the police,’ said he, ’I have killed your master.’
Markheim, then, follows through his decision to hand himself in. He has committed a crime, but he also faces up to it and is willing, in the end, to pay the price. This is the second main event of the story; the first is the murder itself. In between, the story is taken up with the discussion of crime and guilt and such-like themes.
The identity of the visitor is never finally revealed, and remains the most intriguing aspect of the story. In an essential way, this person appears to be some part of Markheim himself; some kind of double or doppelganger, as in stories by Edgar Allen Poe, who influenced Stevenson in the writing of this tale. Stevenson of course is famous for another tale about doubles, Strange Case of Dr Jekyll and Mr Hyde.
The visitor can perhaps most readily be taken as an allegorical projection of Markheim’s conscience. The way that he probes Markheim’s innermost thoughts and emotions is suggestive of this. Most telling is his transformation when Markheim decides to give himself up; his features suddenly become radiant and beautiful, representing, perhaps, the triumph of Markheim's good side over the bad.
In any case, the psychological aspect of the story is the most important one, concentrating as it does upon Markheim’s thoughts and feelings. However, it is dressed up in all the familiar trappings of a gothic supernatural tale: the sensational crime, the gloomy, dusty setting with its ominous ticking clocks, the strange visitor, and the general dark foreboding atmosphere.