The film charts Alexia's trajectory from surviving a childhood crash to becoming a sociopathic motor-show dancer impregnated by an automobile. After embarking on a murder spree with a hairpin and burning down her family home, she violently breaks her nose against a sink and binds her body to impersonate Adrien Legrand, a long-lost missing boy.
She is taken in by Adrien's father, Vincent, an aging fire captain who battles physical decline through steroid injections. Vincent gradually realizes she is not his biological son, but chooses willful denial to fulfill his paternal purpose, while Alexia experiences protective, non-sexual intimacy for the first time.
In the climax, Alexia goes into agonizing labor as black motor oil and metal tear through her skin. Vincent cuts away her clothes, discovering her true physical condition. Comforting her through the fatal birth, Vincent receives the newborn infant, which possesses a fully formed titanium spine. Alexia dies, and Vincent cradles the crying hybrid child against his bare chest, embracing his role as its father.