A young girl named Alexia, played by Agathe Rousselle, creates chaos for her father during a car ride, ultimately leading to a tragic accident that results in a severe skull injury requiring a titanium plate to be inserted into her head. As she emerges from her hospital stay, she distances herself from her parents and develops an unusual obsession with their car.
Years later, Alexia, now an adult adorned with a conspicuous scar on her scalp, performs as a provocative showgirl at a motor show. Her co-worker Justine (Anaïs Fabre) flirts with her, but one night, following a show, a male admirer trails Alexia into the parking lot. In a shocking turn of events, he professes his love and kisses her against her will, prompting Alexia to retaliate violently with a large metal hairpin, killing him.
After this act, as Alexia cleans herself post-murder, she hears a thumping sound echoing from the showroom. She discovers the same car she had modeled with earlier and, in an unexpected moment, enters it unclothed. In a bewildering episode as the car seemingly activates on its own, she experiences climactic pleasure. However, beneath her glamorous life lurks a dark truth; Alexia is a serial killer, having taken the lives of multiple victims over recent months, all while maintaining a dispassionate relationship with her oblivious parents.
As if the turmoil isn't enough, Alexia soon grapples with an unexpected pregnancy. She attempts to abort the child using her hairpin but is unsuccessful. A night at a house party leads to a liaison with Justine, which turns deadly as Alexia murders her mid-act. She continues her spree, dispatching the other guests, except for one who narrowly escapes. In a fit of rage, Alexia incinerates her home with her parents locked inside their bedroom.
Now identified as a fugitive, she drastically alters her appearance, disguising herself as Adrien, a boy who disappeared a decade prior. Vincent (Vincent Lindon), Adrien's father, notices this new "son," but when the police request a DNA test for verification, he instinctively refuses.
Vincent, a fire captain, introduces Alexia to his team at the station, where they're puzzled by the silent, androgynous presence of the supposed Adrien. Despite concerns from others, Vincent's overprotectiveness grows, leading Alexia to ponder escape. Yet when Vincent faces a health crisis due to steroid misuse, she feels compelled to stay.
In a haunting twist, Vincent’s estranged ex-wife visits and encounters the now visibly pregnant Alexia, who refrains from exposing the secret in order to protect Vincent's fantasy. The situation escalates when Vincent inadvertently uncovers a glimpse of Alexia's true form. Instead of turning her in, he chooses to continue his support.
As Alexia’s body deteriorates, revealing mechanical aspects and discharging what appears to be motor oil, she ultimately confides her true identity to Vincent. In a gasp-inducing climax, he assists her in childbirth, but the traumatic experience leads to Alexia's demise. Holding their newborn, whose spine appears composed of titanium, Vincent pours out his grief, whispering, "I'm here."
