Oskar, a meek 12-year-old boy, resides with his mother Yvonne in the western Stockholm suburb of Blackeberg in 1982. His classmates regularly bully him, and he spends his evenings imagining revenge, collecting clippings from newspapers and magazines about murders. One night he meets Eli, who appears to be a pale girl of his age. Eli has recently moved into the next-door apartment with an older man, Håkan. Eli initially informs Oskar that they cannot be friends. Over time, however, the two begin to form a relationship, and exchange Morse code messages through their adjoining wall. Eli learns that Oskar is being bullied by schoolmates and encourages him to stand up for himself. Oskar enrolls in weight-training classes after school.
Earlier, Håkan stops and kills a passerby on a footpath to harvest blood for Eli, but is interrupted by an approaching dog walker. Eli is prompted to waylay and kill a local man, Jocke, making his way home after having said goodnight to his best friend, Lacke. A cat-loving recluse, Gösta, witnesses the attack from his flat but, in disbelief, decides not to report the incident. Håkan hides Jocke's body in an ice-hole in the local lake. Håkan makes another effort to obtain blood for Eli by trapping a teenage boy in a changing room after school. When he is about to be discovered by the boy's friends, Håkan pours concentrated hydrochloric acid onto his own face, disfiguring it to prevent the authorities from identifying him. Eli visits Håkan in the hospital; Håkan offers her his neck for feeding. Eli drains him of his blood, and Håkan falls out the window. Eli goes to Oskar's apartment and spends the night with him, during which time they agree to "go steady", though Eli states, "I'm not a girl".
During an ice skating field trip at the lake, some of Oskar's fellow students discover Jocke's body. At the same time, the bullies again harass Oskar, who hits their leader Conny in the head with a metal pole, splitting his ear. Sometime later, unaware that Eli is a vampire, Oskar suggests that he and Eli form a blood bond, and cuts his hand, asking Eli to do the same. Eli, thirsting for blood but not wanting to harm Oskar, laps up his blood before running away. Lacke's girlfriend, Virginia, is subsequently attacked by Eli. Virginia survives but discovers that she has become painfully sensitive to sunlight. Virginia visits Gösta, only to be fiercely attacked by Gösta's cats. Soon after this, Oskar confronts Eli, who admits to being a vampire. Oskar is initially upset by Eli's need to kill people for survival. However, Eli insists that they are alike, in that Oskar wants to kill and Eli needs to kill, and encourages Oskar to "be me, for a little while."
In the hospital, Virginia asks an orderly to open the blinds in her room. When the sunlight streams in, Virginia bursts into flames. Lacke tracks Eli down to the apartment. Breaking in, he discovers Eli asleep in the bathtub. He prepares to kill Eli, but Oskar interferes; Eli wakes up, jumps on Lacke and feeds on his blood, killing him. Eli thanks Oskar and kisses him. However, an upstairs neighbor is angrily knocking on the ceiling due to the disturbance. Eli realises that it is not safe to stay and leaves that night.
The next morning, Oskar is lured out to resume the after-school fitness program at the local swimming pool. The bullies, led by Conny and his older brother Jimmy, start a fire to draw Mr Ávila, the supervising teacher, outside. They enter the pool area and order the children, aside from Oskar, to clear out. Jimmy forces Oskar under the water, threatening to stab his eye out if he does not hold his breath for three minutes. While Oskar is being held underwater, Eli arrives and rescues him by killing and dismembering the bullies, except for the most reluctant of their number, Andreas, who is left sobbing on a bench.
Later, Oskar is travelling on a train with Eli in a box beside him. From inside, Eli taps the word "kiss" to Oskar in Morse code, to which he taps back "small kiss".[a]
