In 1830, retired veteran detective Augustus Landor is asked by the military to investigate an incident at the United States Military Academy at West Point, New York. Landor is a widower who lives alone since his daughter Mathilde “ran off”, a couple of years previously.

A cadet, Leroy Fry, has been found hanged, but his heart was removed from his body as it lays in the morgue. The senior officers at the Academy wish Landor to find out who did this and why. Examining the corpse, Landor finds a small fragment of a note clutched tightly in the hand of the dead cadet. Also, marks on Fry’s neck and fingers suggest that he did not hang himself, but was murdered.

With the officers’ permission, Landor enlists the help of Edgar Allan Poe, another cadet at the academy who has expressed an interest in the case. Poe and Landor deduce from the writing on the note fragment that it was summoning Fry to a secret meeting. After a cow and a sheep are found in the area, butchered and with their hearts removed, it is deduced that the murder could be linked to black magic rituals.

Another cadet, Ballinger, goes missing and is later found hanged, with both his heart and his genitals removed. A third cadet, Stoddard, who was a colleague of the two victims, then disappears, and it is presumed by Landor that this man had reason to believe he was next in line to be killed.

Landor and Poe begin to suspect the family of Dr. Daniel Marquis, who was first brought into the investigation to perform the autopsy on Fry. Particular suspicion is placed on his son Artemus and his daughter Lea (who suffers from random seizures).

While visiting Dr. Marquis’s house, Landor finds an old officer’s uniform; a man impersonating an officer had been involved in the mutilation of Fry’s body. Landor confronts Dr. Marquis, who admits that he had resorted to black magic to cure Lea of her seizures, and initially she appeared to improve.

Poe is enchanted by Lea and volunteers to do whatever she wants. But he is drugged and wakes to find that Artemus and Lea are about to cut out his heart, in accordance with the ritual to cure Lea. Landor manages to arrive in time to rescue Poe, but the building catches fire and Lea and Artemus die.

Thinking that the case is now solved, the military thanks Landor for his service. However, Poe, recovering from his near-death experience, notices that the handwriting on the note fragment found in Fry’s hand matches that of Landor. Threading together all the information that he has gathered, it becomes apparent that Landor was in fact the killer of the cadets. Poe confronts Landor with his conclusion.

It transpires that two years previously, Landor’s daughter Mattie was raped by Fry, Ballinger, and Stoddard after attending her first ball. Traumatized by the experience, she later committed suicide by jumping off a cliff. Landor did not disclose this to anyone, but pretended that Mattie had run away.

Distraught, Landor set out to avenge his daughter. He left the note for Fry, luring him to a lonely spot before hanging him. But a patrol happened to walk by, so Landor was forced to leave the body there. Lea and Artemus later stole the heart for their ritual. After killing Ballinger, Landor mutilated his corpse to make it appear that the cadet had been murdered by the same “madman” who had desecrated Fry’s body.

Poe tells Landor he has two notes with handwriting samples that can link Landor directly to the murders, but before leaving, Poe burns them. Landor is later seen standing at the cliff where his daughter leapt to her death. He lets her hair ribbon float away in the wind, saying “Rest, my love”.