Oskar, a young boy living with his mother Yvonne in the 1982 suburb of Blackeberg, finds himself the target of relentless bullying at school. As he grapples with feelings of isolation, his evenings are filled with thoughts of vengeance. One fateful night, he encounters Eli, who seems to be a girl his age. Recently moved into the neighboring apartment with a man named Håkan, Eli initially tells Oskar that they cannot be friends. However, as time passes, a bond forms between them, facilitated by messages exchanged through their shared wall.
Eli quickly becomes aware of Oskar's plight and urges him to confront his bullies, which leads Oskar to enroll in after-school classes. Meanwhile, Håkan, desperate to procure blood for Eli, commits shocking acts of violence, including a brutal murder on the street. When things go awry, he resorts to drastic measures that result in tragedy, including an ominous encounter with Eli in the hospital, where she ultimately feasts on his blood, leading to his demise.
Their emotional connection deepens one night when Eli spends the evening at Oskar's apartment, and they make a poignant commitment to each other, even as Eli reveals, “I’m not a girl.” Unfortunately, Oskar's struggles persist when the bullies discover a horrific secret relating to Jocke's body. After suffering another attack, Oskar retaliates against the bully, Conny, and offers his own blood to Eli, which she consumes eagerly.
A dramatic turn occurs when Virginia, who is inadvertently drawn into Eli's world, finds herself at the mercy of sunlight and tragically meets her end. Oskar ultimately confronts Eli, who confesses her true nature as a vampire. Despite the unsettling truth, Eli suggests that they share a dark bond—both needing to kill in their own ways.
As events spiral, Lacke tries to hunt Eli down but is thwarted when Oskar intervenes, resulting in Eli expressing her gratitude and affection in a kiss. With an impending disturbance from the neighbors, Eli realizes their time together is drawing to a close, prompting her to leave unexpectedly.
In a climactic finale, Oskar is forced into a terrifying situation with his bullies at the pool, where Eli heroically intervenes to protect him, leaving the bullies in a state of disarray. The film wraps up with a touching moment between Oskar and Eli on a train, where they communicate a tender message in coded taps—"kiss"—signifying their deep connection amidst the chaos that surrounds them.
