In a snowy suburb of Stockholm during the 1980s, Oskar, a sensitive 12-year-old boy, struggles with bullying at school and a fractured home life. With his father living elsewhere and his mother distracted, Oskar spends his days dreaming of revenge on his tormentors, escaping into morose fantasies and collecting newspaper clippings about local murders. He lives a lonely, detached existence until he meets Eli, a mysterious girl who has recently moved into his apartment complex with an older guardian. Eli is peculiar and reclusive, her presence shrouded in enigma. Despite an initial warning that they cannot be friends, a subtle bond forms between the two solitary children.

As the friendship deepens, a series of brutal and unexplained killings spread fear throughout the neighborhood, conspiring with the harsh winter to cast a shadow over the community. Eli encourages Oskar to stand up for himself, which leads to a disturbing revelation about her true nature. The connection between the children grows, fueled by their mutual needs for companionship and acceptance, yet underpinned by a darker truth that begins to surface. Oskar is confronted with the reality that Eli's arrival and the ensuing bloodshed are intertwined, forcing him to grapple with moral ambiguities and the perils of an innocent love blossoming under the specter of a grim secret.