Rory Morris, a 27-year-old Manhattan business consultant, has temporarily returned to her unnamed hometown to support her pregnant twin sister, Scarlett, a tattoo artist who told Rory that her boyfriend, Matty, abandoned her. One foggy October night, Rory runs into Ian Pedretti, a bespectacled former classmate who has had a crush on her since middle school. Driving home, she finds the remains of a deer on the road and is confronted by a massive creature: upright, gray-furred, with elongated arms, frothing jaws, and vivid red eyes. It chases her into the woods and bites deeply into her side. Another animal distracts it, and Rory passes out. A jogger finds her the next morning.
At the hospital, Rory claims a bear attacked her. Her injuries are puzzlingly minor despite the devastating pain she experienced. At home, she discovers iridescent silver pus oozing from her wound and glimpses the creature in her bathroom mirror.
Over the following weeks, alarming changes accumulate: obsessive meat cravings, abnormally thick hair, and a wound that continues to seep silver. At a costume shop, Rory sees a werewolf animatronic resembling the creature and confirms online that it was a full moon the night of the attack. At a Halloween party hosted by Rory and Scarlett's childhood best friend, Ashley (Ash), Rory and Ian share their first kiss. Inside, she accidentally sends a man flying with a light push, revealing superhuman strength. She reconnects with Mia Russo, a charismatic high school friend who is back in town after her father's death. Later, Rory discovers coarse gray fur growing beneath her wound, and a silver choker burns her neck when she tries it on. The evidence is mounting: She has been turned into a werewolf.
Rory begins dating Ian, who is direct about wanting more than a hookup. She stays at his apartment, a renovated 1898 bank building with a vault in the basement. Meanwhile, Scarlett reveals the baby is a girl and that their mother, Cyndi, is visiting soon, which fills Rory with dread.
Rory's relationship with Cyndi is rooted in unresolved childhood trauma. When Rory and Scarlett were 11, Cyndi brought a boyfriend named Dave into their home. Rory disliked him immediately, but Cyndi dismissed her instincts. Dave singled Rory out with gifts and solo outings, escalating to inappropriate touching and exposing himself to her. Scarlett responded with doubt when Rory told her. Cyndi refused to listen. Rory reported Dave to her school guidance counselor, who believed her and took action. Dave was already a registered sex offender. Cyndi has never apologized.
On the first full moon since the bite, Rory locks herself in a dog crate at an abandoned distillery, handcuffed. Memories of Dave flood her. The transformation is excruciating: Her skin splits, bones break and reform, teeth fall out and regrow, and fur erupts. She hesitates before cuffing herself, frozen by the same helplessness she felt as a child. She breaks free easily and loses herself to the wolf, waking naked in a ditch at dawn.
Cyndi's visit brings tense confrontations. Cyndi wants forgiveness but refuses to admit fault. After a surprise baby shower for Scarlett, Scarlett goes missing, and Rory and Cyndi argue while searching for her. That evening, Rory confides in Scarlett, dragging a screw across her forearm: Silver blood oozes out with sprigs of fur before the wound heals. Scarlett responds with doubt, suggesting Rory see a therapist for "clinical lycanthropy," a condition in which a person believes they can transform into an animal. Rory is devastated by her sister's disbelief.
Scarlett is later rushed to the hospital with severe pains, a false alarm caused by dehydration. In the waiting room, Rory encounters Matty and accidentally dislocates his shoulder. Matty reveals he did not abandon Scarlett: She kicked him out. Rory reconciles with Ian after ignoring his messages for over a week. At his place, she opens up about her family struggles. Ian tells her he wants to be with her but needs her to communicate, not disappear. Rory later tells Cyndi she forgives her, recognizing the forgiveness is for herself.
Before the second full moon, Rory records her transformation to prove it to Scarlett. The video works, and Scarlett's doubt finally vanishes. Scarlett confesses that she cheated on Matty, panicked, and threw him out rather than admit the truth. They are now working through it. Rory also experiments with a silver bracelet, which dulls her senses but renders her unconscious for three days, teaching her that silver has power over her condition but must be handled carefully.
At a girls' night out, Mia mentions she went out with Ian and he has been texting her. On New Year's Eve, Ian explains he only took Mia for a drink after her father's funeral. Nothing happened. Inside the party, Rory catches a familiar scent: the wolf that bit her. She recognizes Mia's boots as the pair she lost the night of the attack. Rory erupts: "It was you!" Silver blood appears on Mia's scalp during the altercation. Mia insists the bite was accidental, that she lost track of the lunar cycle. She flees. Rory collapses crying and tells Ian, "I'm falling in love with you and I'm a fucking monster!" At midnight, they kiss and reconcile.
A vial of tonic appears on Scarlett's doorstep with a note from Mia. Made from wolfsbane, a toxic plant, it weakens the wolf if taken daily for six days before the full moon. The side effects are brutal. Rory also crafts a silver sun-shaped medallion on a leather cord, long enough to rest on her clothing rather than burn her skin. When Scarlett goes into labor, Rory arrives at the hospital barely functional from the wolfsbane. The baby, named Lennox, is born. Rory holds her niece and whispers, "I've got you, kid."
Days later, driving with Ian, Rory realizes the full moon is tonight. She runs into the woods and tells him: "I was attacked by a werewolf. And now I am one." The transformation begins in front of him. In wolf form, the silver medallion rests against her fur over her heart, and its calming effect radiates through her body. Her thoughts clear. She is in control. Another wolf appears: Mia, unrestrained, who attacks Ian. Rory intercepts. She realizes, "There is no me and the wolf. I am the wolf." She pins Mia down until sunrise.
At dawn, Mia reveals she no longer wants to control herself, seeing the wolf as freedom from the powerlessness she endured at her abusive father's hands. Rory delivers an ultimatum: Leave, take measures to control yourself, and never come near my family, or I will kill you. That evening, Ian walks up Scarlett's driveway in yesterday's clothes, glasses cracked, having spent the day searching for Rory. She offers him an out. He refuses: "I don't care. That you're a werewolf. I don't care." She warns him, "I have such sharp teeth." He kisses her anyway. A postcard arrives from Mia in Texas: She is keeping her promise, "for now."