Tsukasa, a young man drowning in debt and caring for a chronically ill younger sister after his father abandoned the family, makes a calculated decision. He plans to take his own life and disguise it as an accident, ensuring his family receives a life-saving insurance payout. His plan succeeds, but instead of oblivion, he awakens in a torture chamber. He is no longer himself; he has been reincarnated as Heinreid von Rodvelia, the most sadistic and despicable duke from a light novel he once read. The world is a brutal fantasy setting of nobles and kingdoms, and Heinreid is its worst tyrant. Worse still, Tsukasa is not a simple replacement. Heinreid's thoughts and memories remain, forcing Tsukasa to experience every horrific act of violence, torture, and depravity the duke committed as if he had done them himself.

Haunted by the borrowed guilt of Heinreid's atrocities, Tsukasa resolves to follow the original story's plot to its grim conclusion: his own death at the hands of Craig Bauer, his personal guard and the novel's hero. Before that end, however, he decides to use his remaining time to settle the duke's affairs, secretly righting wrongs, saving innocent lives that were meant to be destroyed, and preparing for his execution as a form of penance. But his sudden shift from a monster to a quiet, selfless man does not go unnoticed. Craig, who was secretly plotting the duke’s downfall, becomes intensely suspicious of this new, gentle Heinreid. His mission to kill his master blurs into an obsessive fascination, and his feelings begin to evolve in a way that threatens to derail the entire original storyline.

Just as Heinreid believes his plans for atonement and death are on track, the narrative pivots on a crucial confrontation. The king sees through Heinreid's altruistic actions, accusing him of a profound selfishness and cowardice. The king forces Heinreid to recognize that his grand plan to die a martyr is merely an elaborate escape, a parallel to the suicide he committed in his original life. It is an attempt to run from the pain of living and the responsibility of true atonement, not a heroic sacrifice. This shatters Heinreid’s worldview. As Craig’s obsession grows into devotion, Heinreid is forced to choose between his doomed destiny and a life he believes he does not deserve. The story concludes on a bittersweet note; while Heinreid abandons his plans to die, he cannot bring himself to seek happiness. He resolves to live, not for joy, but for eternal atonement, leaving his relationship with Craig in a complex and painfully earned place rather than a purely romantic one.