In the rolling hills of 19th-century Pennsylvania, Sin Eater weaves together the lives of two unconventional families, exploring themes of love, redemption, and the weight of secrets across generations. Sin Eater centers on Osborn Roche, a neuro-divergent, renowned Civil War photographer who retreats into isolation following his wife's death. He assumes the role of a "sin eater," a person who ritualistically consumes the sins of the deceased, providing him a unique position in society.

Parallel to Osborn's journey is the tale of the Fenn family, particularly Charlotte Fenn, a "weeper" who professionally mourns at funerals. Both find their lives intertwined in ways they never imagined. As they navigate the complexities of family, forbidden love, and long-buried truths, they discover that redemption can come from the most unexpected places. As the plot unfolds, it reveals the intricate connections between the Roche and Fenn families, particularly through Osborn's son Ishmael and Charlotte's granddaughter Dierdre. Their budding romance is complicated by family interference and misunderstandings, leading to years of separation and setting off a chain of events that forces long-buried secrets to surface, challenging the characters to confront their past decisions and their consequences.

Set against the backdrop of a changing America, from the aftermath of the Civil War to the dawn of new technologies like the telephone, Sin Eater paints a vivid picture of a society in transition. It delves into the customs and superstitions of the era, particularly the practices of sin eating and professional mourning, using these as lenses through which to explore deeper human experiences of grief, guilt, and redemption.

With rich historical detail and unforgettable characters, Sin Eater is a tale of love that defies societal norms, family secrets that span generations, and the healing power of acceptance.