Premise

The film takes place in the town of Bailey Downs, Canada, in October 1999. During this time period, an unknown beast nicknamed the "Beast of Bailey Downs" has been killing the town's dogs.

Brigitte, the second daughter of Pamela and Henry Fitzgerald, is a bright 15-year-old with a love for biology, having skipped a grade. She and her sister Ginger share a close bond, growing up inseparable. Both are introverted nihilists, disdainful of modern society and fascinated by death. Their unconventional views and delayed onset of menstruation isolate them from peers. At eight, they made a blood pact to be "united against life," vowing to move out at 16 or face death together. Brigitte looks up to her older sister Ginger, who assumes a leadership role despite occasional ridicule. The sisters have a strained relationship with their parents, who play a passive and irritating role, effectively absent as guiding figures.

Story

The film begins on an atypical day in high school for Brigitte, finding out that Ginger, her sister, has started her period. That night on a full moon, Brigitte rescues Ginger from a beast attack. Observing Ginger's unnatural rate of healing from the attack and not knowing what attacked them, with no answers and no other choice, Brigitte breaks with her "introverted" persona to connect with local high-school drug dealer Sam (who ran over the beast chasing the sisters, saving them) for more information; he suggests the beast might be a lycanthrope.

Brigitte discovers physical and mental changes in Ginger, reminiscent of puberty, such as hair sprouting from wounds and heavy menstruation. She is dismayed by Ginger's flashy appearance and aggressive, particularly sexual, behavior. Like her sister, Brigitte begins to "grow up" and gains confidence. However, she starts antagonizing Ginger for embracing the changes, conflicting with their childhood pact to be "united against life." Brigitte becomes more distant from her sister and finds new company in Sam. After learning about Ginger killing a neighbor's dog, Brigitte tries a proposed silver ring piercing treatment by Sam, which proves ineffective.

On Halloween, Brigitte obtains monkshood from her mother and seeks further help from Sam. Meanwhile, Ginger, angered by what she perceives as Brigitte's "betrayal" and the belief that Brigitte has chosen Sam over her, refuses to assist Brigitte in the future. Brigitte discovers that Ginger takes pleasure in her violent impulses after Ginger murders two school faculty members, leading to a confrontation where Brigitte berates Ginger. To Brigitte's dismay, Ginger makes a definitive decision, revokes their childhood pact, and declares she no longer wants anything to do with Brigitte, leaving her behind.

Later at the Greenhouse Bash Halloween party, Brigitte discovers Ginger harming Sam, who rejected Ginger's sexual advances. At her breaking point, Brigitte insists that Ginger take her life instead to stop hurting everyone. Determinedly, Brigitte inflicts a palm wound on herself and Ginger, mixing the two girls' blood to signify Brigitte's loyalty to her sister. Brigitte and Sam take Ginger, who is rapidly undergoing the final stages of transformation into a werewolf, to the Fitzgerald house to create more cure. Brigitte is seen feeling numb and later experiences a fainting episode, indicating her beginning transition as well.

At the Fitzgerald house, a fully transformed Ginger escapes, but Brigitte and Sam successfully create more cure in the kitchen closet amidst the turmoil. Sam proposes to Brigitte to take the cure herself, and the two of them run away together, but Brigitte, despite also showing affection for Sam, values her loyalty to her sister more and refuses his request. Sam goes to administer the cure but is dragged away. Brigitte goes after them and finds a Ginger-wolf with a weakened Sam. Trying to prove her loyalty to Ginger, Brigitte starts to drink Sam's blood but can't continue, making Ginger, in a fit of rage, kill Sam.

Cornering Ginger in the sisters' bedroom, Brigitte, at last realizing that Ginger never appreciated her and knowing that Ginger took away Sam from her, fully breaks away from the sisters' childhood pact to be "out by sixteen or dead in this scene, but together forever" and defiantly states that she will not die in the room with her. Ginger, angered by Brigitte, lets her werewolf instincts take over and lunges toward Brigitte. Brigitte accidentally kills Ginger as the werewolf lunges into her knife. The film ends with Brigitte holding on to the dying Ginger and unused syringe and sobbing.