Let the Right One In, a novel by John Ajvide Lindqvist, tells the story of Oskar, a lonely and bullied 12-year-old boy living in a suburban Swedish town in the early 1980s. Oskar befriends Eli, a mysterious girl who moves into his apartment complex. Eli is revealed to be a vampire, appearing youthful but carrying the burden of centuries of life and experience. 

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Their bond deepens as Eli helps Oskar gain confidence and stand up to his tormentors. Meanwhile, a series of gruesome murders begin to plague the town, committed by Eli's caretaker, Håkan, who procures blood for Eli to survive. Håkan's increasingly reckless actions draw attention to Eli, putting both their lives at risk.

As their friendship grows, Oskar learns of Eli's true nature but accepts her for who she is. In return, Eli protects Oskar, ultimately saving him from his bullies in a brutal confrontation. The novel culminates with Eli fleeing the town to avoid capture, leaving Oskar behind.

However, Oskar decides to join Eli, suggesting that their bond transcends the boundaries of human and vampire, loneliness and companionship, offering a unique and poignant exploration of love and loyalty.