Set in 1830 at the United States Military Academy at West Point, the novel is narrated by Augustus "Gus" Landor, a retired New York City constable living near the Academy in Buttermilk Falls. In a prologue dated April 1831, an ailing Landor announces he is hours from death and dedicates the manuscript to an unknown future reader.
The main narrative begins in October 1830, when Landor is summoned to the Academy by Superintendent Sylvanus Thayer and Commandant Ethan Allen Hitchcock. A second-year cadet named Leroy Fry has apparently hanged himself, but during the night someone stole the body from the hospital and surgically removed his heart. Thayer needs a discreet civilian investigator, as the Academy's political enemies could exploit the scandal. Landor accepts the unpaid commission on condition that he move freely and report only to Hitchcock.
Examining Fry's body, Landor discovers evidence of murder: a skull contusion, blisters on the fingertips from fighting the rope, and lacerations indicating prolonged struggle. He also finds a torn scrap of paper in Fry's clenched hand. Witnesses reveal that Fry headed out on private "Necessary business" the night he died and had once fallen in with a "bad bunch" before briefly joining the Academy's prayer squad.
In the superintendent's garden, a young plebe, or first-year cadet, named Edgar A. Poe approaches Landor uninvited and announces, "The man you're looking for is a poet." At Benny Havens' tavern, a local establishment forbidden to cadets, Poe argues that removing a heart is an act of symbolic significance and demonstrates sharp powers of observation. Impressed, Landor persuades Thayer and Hitchcock to let him recruit Poe as a covert agent within the corps, despite objections to Poe's low standing and disciplinary record. Poe accepts, declaring himself "perversely honored." Privately, Landor writes to Henry Kirke Reid, a New York investigator, requesting a background check on Poe.
Together, Landor and Poe decode the scrap of paper as an invitation to meet at a cove near the river landing, theorizing Fry believed he was meeting a woman. A newspaper from nearby Haverstraw reports farm animals killed with their hearts removed. Near the icehouse where Fry's heart was taken, Landor maps holes in the ground forming a circle with an inscribed triangle; scorch marks reveal the reversed Christian monogram JHS, suggesting diabolical inversion. A reclusive scholar named Professor Pawpaw identifies the markings as a magic circle and connects them to Henri le Clerc, a 17th-century French priest who converted to devil worship and left behind a legendary occult text called
Discours du Diable. A related text notes that sabbath rituals call for "the hearts of hanged men."
Poe infiltrates the Academy's prayer squad and learns that Artemus Marquis, a senior cadet and the Academy surgeon's son, is reputed to dabble in dark arts. Through calculated social maneuvering, Poe gains entry to the Marquis family home, where he meets Artemus' volatile mother, Alice Marquis, and his sister, Lea, a 23-year-old known as the "Sorrowing Spinster." Poe falls deeply in love with Lea, who wears orrisroot perfume reminiscent of his dead mother. During a meeting in the cemetery, Lea collapses in convulsions; Landor privately recognizes the symptoms as epilepsy, the condition that has made her unmarriageable. Despite this, Lea and Poe declare their love.
The danger escalates. A cow's heart and a bomb are found in Artemus' barracks, though Artemus has an airtight alibi. Cadet Randolph Ballinger, Artemus' roommate, assaults Poe in a stairwell. Days later, Ballinger is found hanged, naked, with his heart removed and his body castrated. Hitchcock confronts Landor with evidence that Poe threatened Ballinger and also had a hidden conflict with Fry. At the Marquis home, Landor discovers a bloodstained officer's uniform in Artemus' closet and narrowly survives an attack by an unseen assailant. Stoddard, the last witness to see Fry alive, flees the Academy. Hitchcock removes Poe from the case and gives Landor three days before a replacement investigator arrives.
To sever his bond with Poe and prevent Poe from ever suspecting the truth, Landor uses Reid's background report to systematically demolish Poe's self-mythology, exposing fabrications about his past and mocking his family. Poe departs devastated, saying, "You will one day feel what you have done to me." Separately, Landor physically confronts John Allan, Poe's foster father, who has come to West Point to disown Poe.
Lea begs Landor to stop investigating Artemus, then suffers a seizure and throws herself from the bluff at Kosciusko's Garden; Landor catches her and hauls them both to safety. As she revives, she whispers, "He said it will all turn out," the same words Landor's daughter Mattie spoke before her own death. Poe announces his engagement to Lea. Landor identifies a portrait in the Marquis home as Henri le Clerc. Confronting Dr. Daniel Marquis, Landor learns the family's secret: Lea believes she communicates with le Clerc during her seizures, and le Clerc promised a cure through ritual. The family became complicit: Artemus as surgeon, Mrs. Marquis as participant, and the doctor through willful ignorance. When Landor notices Poe's cloak still in the front hall, he realizes the family has drugged Poe and taken him to the icehouse.
Landor falls through the icehouse floor into a hidden underground chamber where Artemus, Mrs. Marquis, and Lea are performing a ritual within a magic circle. Poe lies drugged on the floor, his arm cut open, blood dripping into a vessel; the ritual requires virgin blood. Lea pours the blood over her head and opens a box containing Fry's heart. Landor intervenes by exploiting Mrs. Marquis, forcing her to choose which child to save. Artemus confesses to the murders and names Stoddard as his accomplice. Before Landor can seize the heart as evidence, Lea swallows it and begins to choke. Artemus attempts an emergency tracheotomy but severs an artery, and Lea bleeds to death. Artemus stabs himself. The blazing brazier ignites the wooden ceiling, and the ice store above crashes through in massive blocks, crushing Artemus as he cradles Lea's body. Landor drags Poe and Mrs. Marquis to safety.
In the aftermath, Mrs. Marquis loses her sanity and Dr. Marquis resigns. Poe, recovering from blood loss, returns to Landor's cottage and announces, "I know about Mattie." He has discovered that a mysterious poem contains an acrostic spelling "MATHILDE DIED" and has matched Landor's handwriting to the note from Fry's hand. Patsy, the barmaid at Benny Havens' and Landor's lover, confirmed the hidden story: Years earlier, Fry, Ballinger, and Stoddard raped Landor's daughter Mattie. She never recovered and threw herself from a bluff. Consumed by vengeance, Landor lured Fry to the landing, killed him, and removed his heart to make the murder resemble satanic activity. He practiced on farm animals and planted the cow's heart in Artemus' barracks to frame the Marquis family. He later killed and castrated Ballinger. The family's occult activities provided cover for Landor's own crimes, and he manipulated the entire investigation to conceal his guilt. Landor offers Poe a loaded flintlock and asks to be shot. Poe refuses, calling it the coward's way, and departs.
In the months that follow, Poe engineers his own dismissal from the Academy by refusing to attend any duties. He sends Landor a newspaper clipping reporting Stoddard's death by hanging in New York after a visit from an unidentified man. He also sends a copy of
Poems by Edgar A. Poe with an epigraph from La Rochefoucauld: "Everybody is right." Landor, alone and visited by ghosts, finishes his confession, hides the manuscript, and walks to the same bluff from which Mattie jumped. He addresses his daughter directly, asking if she will be waiting, and the narrative ends.