In March 1983, the serene locale of Los Alamos, New Mexico, is shattered when a group of police cars, their sirens blaring, rush an ambulance to the Los Alamos Memorial Hospital. Inside, they carry an unidentified 50-year-old man, his face marred by acid burns. As he is placed on a respirator due to the toxic fumes having scorched his lungs, a detective, Elias Koteas, attempts to elicit a response from him, questioning if he belongs to a Satanic cult—but he receives no answer. In a twist of fate, the man suddenly escapes from the tenth floor window, leaving behind a haunting note that reads, "I'm sory [sic] Abby".
Fast forward two weeks, Owen (portrayed by Kodi Smit-McPhee), a troubled twelve-year-old, finds solace in the snow-covered courtyard of his apartment as he sings alone, often left to his own devices due to his parents' tumultuous divorce. One fateful evening, he notices a new girl, Abby (Chloe Moretz), moving in next door with a peculiar man (Richard Jenkins) who is not truly her father. Abby is a mysterious figure—she is barefoot and exudes an unusual aura.
School is a battleground for Owen, who frequently becomes the target of older bullies. One particularly humiliating day, he fails to defend himself against a vicious attack, resulting in a deep-seated fear that lingers. As he practices retaliatory knife strikes in the courtyard, he catches Abby watching him, her demeanor cold as she asserts they cannot be friends. Little does he know that Abby harbors a dark secret—she is not who she appears to be; she is indeed a vampire, and her so-called father is merely her "renfield," bringing her sustenance through heinous acts.
As their friendship blossoms, Abby displays an alarming hunger—a hunger caused by her father’s increasing inability to secure blood for her. One night, after an alarming incident involving a local woman, Abby’s desperation reaches a peak. Meanwhile, Owen begins to shield himself against his tormentors, encouraged by Abby to stand up for himself.
Intriguingly, during an ice-skating outing, Owen defies his bullies for the first time, asserting his strength. However, it’s all seemingly trivial in the grand scheme of Abby's bloodlust, which spirals out of control as Father succumbs to his tragic fate. Abby, upon encountering Owen in a vulnerable state, reveals her true vampiric nature, leaving behind her old truths and diving into a blood-soaked existence.
Abby’s struggle to maintain her humanity leads her to confront dark choices, ultimately resulting in a reckoning with her past that culminates in violence. When Owen discovers the depths of Abby's reality, he is torn between his growing affection for her and the terror she instills in him.
In a shocking finale, as Abby re-emerges to save Owen from certain doom at the hands of bullies, she leaves a mark on his life that he cannot escape. The epilogue sees Owen aboard a train, snow falling outside, clutching a large trunk that signifies a new chapter. Their futures intertwined, Owen taps out Morse code responses from within the trunk, affirming his role as Abby’s protector, as they embark on an unforeseen journey toward a new life.
