In 1982, 12-year-old Oskar stays with his Mom Yvonne in the Blackeberg neighborhood of Western Stockholm. Oskar witnesses a man named Hakan and his daughter Eli exiting their car and entering the apartment where he lives. Another neighbor from the same street, Lacke, stands close to the building and looks up, noticing the man sticking the cardboard to the window glass. Oskar is frequently teased by his peers at school. When walking home from school, he glances at the windows of the new neighbor before leaving. Hakan is getting ready, gathering his belongings, and preparing to leave. Hakan then spots the trespassers while standing beneath the streetlight. He picks up a stranger, pretends to ask him for his time, and knocks him out. After killing him, he slits open his throat and collects the blood in the can.
Unexpectedly, the dog of a trespasser notices him in action and alerts his owner by barking at him. As soon as they get there, Hakan flees. Oskar reads murder-related material in his spare time and plans his revenge at night to teach his friends a lesson. Eli, who seems to be a pale girl of his age, enters his life the same evening. Eli asks and shares her whereabouts. Eli initially tells Oskar that they are not suitable buddies. As Hakan boards the train, he looks over his belongings and notices that his blood bag is missing. Eli yells at him when he gets home, wondering if she has to watch out for herself. Due to the strange killings of kids happening, Oskar's mom asks him to reach home once school is over. He reviews the missing person's cases and compiles the magazine snippets in his journal.
Next, Lacke joins his wife and his colleagues at a restaurant. They also notice Hakan at the nearby table, also a newcomer to the street, and Lacke tries to talk to him, but he leaves Lacke unresponsive. Oskar makes a false explanation before leaving his house to meet Eli. While he is solving the Rubik's cube, Eli arrives and sits behind him, and they begin to converse. He acknowledges that she smells strange, and she attempts to say that she hasn't experienced the cold in years. She attempts to solve the Rubik's cube after getting it. She experiences appetite loss. Following their meeting, Lacke and his colleague Jocke wish to depart for their homes.
While Jocke walks beneath the bridge, he sees a young child named Eli sitting in a corner and pleading for help. Jocke lifts her when he approaches to support her; Eli bites him suddenly and begins draining his blood, then leaves him to die. She regrets it and sobs as she leans over his lifeless body. Gosta, a man who lives in the opposite Centrum Building with his cat, sees what has jolted him and enters his room silently out of horror. Later, he tells his colleagues about the murder at a bar. He leads them to the scene and displays Jocke's blood footprints. Parallelly, Eli tells her father about the killing she committed. Hakan yells at her behavior.
Hakan quickly makes his way to the crime scene and takes the body from the area. Then he places the dead body in the enclosed sewage pool. After finishing the cube and leaving it at their meeting spot, Oskar grabs it the following morning and waits for her that evening for a visit. When Eli approaches and asks if she smells better now, he says yes. Her birth date was requested by Oskar. Eli claims that she cannot recall the occasion and that she has never gotten a birthday gift. Oskar begs her to retain the cube as a gift from him, and, out of enthusiasm, he figures out how to solve the cube. The following day, he stays late in class and self-learns Morse code to teach Eli and use it as the basis for discussions between them.
The same evening, on the way home from school, he was bullied again by his friends. Then he meets her and teaches her the Morse code to initiate their communication through it. When Eli finds out that Oskar is a victim of bullying at school, she urges him to defend himself. Then the two eventually start to get along and start communicating through their shared wall in Morse code. After school, Oskar signs up for weightlifting classes. He brings her to the candy store, where he purchases some mixed candies. Eli crosses the shop, causing the cat to act rigidly. Oskar gives Eli the candy.
She initially ignores him, but she gives in when he begs for one. She started to throw up as a result of it. After hugging her, he makes his apology, and she asks him if he still likes her even though she is not a girl. He replies, "Yes." Eli is cautioned by Hakan to stay away from Oskar. Hakan watches some young kids playing basketball. He cares about the ideal time for the kids to leave home. He picks out one boy, Matte, whom he catches, and attempts to slice open his throat. His friends enter to see how he is doing. When Hakan discovers his teammates, they head inside to look at him. He immediately enters the adjacent room and pours acid on his face to defend himself from being beaten to death. Eli is found waiting for Hakan to bring the blood bag to their residence. Oskar attends his weightlifting class and discovers that his clothes have been drenched in the toilet as a result of bullying from his classmates.
Hakan is now a patient at the medical facility. Eli stops by the hospital to see how he is doing. The hospital's receptionist refused to let her inside to see him because of the case that had been filed against him. Then she employs a smart vampire trick to spy on him. He notices her in the windows and happens to come in front of her. He offers himself as a sacrifice to satiate her appetite. She then pays Oskar a visit to his house. He requests that their connection remain constant. Eli, however, calmly requests that we accept things as they are at this time. Together with his classmates, Oskar is taking an ice skating field trip from his school. Some of Oskar's classmates find Jocke's body while on an ice skating field trip to the lake.
Parallelly, When the bullies start abusing Oskar again, he strikes them with a metal stick, tearing Conny's ear in half. On the other side, two young boys discover a dead body in a sewage pool. They informed their teacher, which led to a police investigation. As a result, the police conclude that this is the case of Jocke, who has been missing for some time. Upon hearing about this, Yvonne advises him against his actions. He then takes Eli along as he goes to his swimming lesson. She observes him through the mirror. Later, Oskar proposes that he and Eli make a blood bond and slashes his hand, urging Eli to join in. At this point, Oskar is unaware that Eli is a vampire. Eli sips Oskar's blood droplets from the floor before fleeing because she is bloodthirsty but doesn't want to hurt him.
Later on, Gosta and his friends are having a conversation in the pub. Gosta, the lone witness to Jocke's murder, feels terrible for remaining unmoved despite having witnessed the crime. Gosta was being comforted by Lacke. Lacke shouts at Virginia when she interrupts, which makes her feel hurt. She exits the bar right away, and Lacke follows her to comfort her. Eli suddenly leaps from the tree, bites her neck, and drains her blood. She is blood-starved. Lacke moves closer to them and kicks Eli to defend her. Eli quickly leaves the area. Virginia makes it out alive, but she soon learns that the sunshine now makes her excruciatingly sensitive.She also discovers that she is growing to crave the taste of her blood.When Virginia visits Gösta, his cat viciously attacks her. Following that, she was checked into the hospital. Soon after, Eli is confronted by Oskar, who acknowledges that she is a vampire.
Oskar is initially infuriated by Eli's requirement for murder to survive. In the hospital, Virginia requests that an orderly open the curtains in her hospital room. Virginia catches fire when the sun shines in Jimmy, Conny's cousin, comes to see him at school. Eli pays him her first visit to his house through the front entrance that evening. Eli awaits his invitation to let her in. He continues to welcome her inside with gestures despite his refusal to verbally invite her in. When she enters, blood begins to seep from all of her body's exposed pores. Oskar then understands the consequences of what she has to endure when she is not invited in. For a while, Eli asks him to assume himself as one like her, where Eli is pictured at her senior age.
Eli cleans the blood and takes a bath. Oskar enjoys the music in the meantime. In exchange for dresses, Oskar provides his mother’s clothing. When she is getting dressed, he peeks in and sees her female organ. He is surprised by how different it is from women's. As his mother rings the doorbell, she exits through the window and goes to her apartment. The following day, Lacke goes to the scene of Jocke's murder. In a parallel scene, Oskar reads Eli's notes, which state that she is in the bathroom and not to be disturbed, and to catch up with her tonight if desired.
Eli is followed to the flat by Lacke. When he sneaks inside, he finds that the windows have all been covered to keep out the sun. He breaks into the bathroom door and finds Eli dozing in the bathtub. Lacke gets ready to murder Eli, but Oskar gets in the way. Eli then wakes up, leaps on Lacke, and consumes his blood, drowning him to death. Oskar receives a thank-you kiss from Eli. Unfortunately, the uproar has caused an above-floor neighbor to angrily knock on the ceiling.
Eli decides that night to depart after realizing it is unsafe to stay. He pursues the car until it vanishes from view. He then goes to her empty apartment and cries all night. Through a false call, Conny and his friends entice Oskar to attend the swimming practice. Oskar is coaxed outside the following morning to continue the after-school training program at the neighborhood pool. The bullies light a fire to entice Mr. Vila, the instructor in charge, outside. Conny and his older brother Jimmy serve as the bullies' leaders.
Except for Oskar, they tell the kids to leave the pool area as they enter. Jimmy drags Oskar below and demands that he breathe deeply for three minutes, or else he will stab his eye out. Eli shows up as Oskar is being held underwater and frees him from murdering and mutilating bullies by killing them cruelly. All but the most recalcitrant Andreas are left crying on a bench. Later, Eli is in a box next to Oskar as they travel on a train. Eli taps the word "kiss" to Oskar from within and taps back "little kiss" in response.
