The story begins with an extraordinary development from the SETI (Search for Extra Terrestrial Intelligence) program, which in 1974 sent out signals sharing details about Earth and its life forms, including DNA structure, in hopes of making contact with extraterrestrial entities. Two decades later, Earth receives an intriguing communication from an alien source. The first message reveals a method for creating renewable fuel using a methane catalyst, leading scientists to believe the source is benevolent.
However, the second transmission takes a darker turn, presenting details about an alien DNA sequence and instructions on how to splice it with human DNA. This prompts a government initiative under the leadership of Xavier Fitch to embark on a perilous genetic experiment aiming to induce a female with traits deemed "docile and controllable." The experiment yields a girl named Sil, who presents as a typical human but matures rapidly, attaining the age of 12 within just three months.
As Sil begins to exhibit violent tendencies, especially during her nightmares, the scientists begin to see her as a potential threat. In an attempt to neutralize her, they resort to cyanide gas, only for Sil to escape her confinement, prompting the government to gather a specialized team. This group includes an anthropologist, Dr. Steven Arden, a molecular biologist, Dr. Laura Baker, empath Dan Smithson, and mercenary Preston "Press" Lennox to track her down and eliminate her.
Sil’s journey takes a horrifying turn as she dreams of being chased by an alien train, triggering a transformation within an organic cocoon that has formed around her. After killing a train warden, she emerges as a striking blonde woman in her twenties, completely naked. The ambiguity surrounding her nudity raises questions about the cocoon’s effects—whether it destroyed her clothes or if she intentionally discarded them.
Arriving in Los Angeles, Sil proves to be extraordinarily powerful and intelligent, posing a significant danger. The scientists fear she may seek to reproduce, potentially leading to the extinction of humanity, given her brutal methods used against anyone obstructing her path. In her quest to find a mate, Sil first seduces a man at a nightclub, only to kill him in a fit of rage upon discovering he is diabetic. A later encounter after a car accident is similarly disastrous, interrupted by Press and Laura. Sil’s transformation into her true form, a terrifying humanoid creature with tentacles, results in the death of another man.
In a state of desperation, Sil flees into the forest, devoid of clothing. After a period of hiding, she cuts and dyes her hair, developing a fascination with Press. She cleverly seduces Dr. Arden, leading to a passionate encounter that results in her pregnancy. However, upon his realization of her true identity, Sil kills Arden, desperately fleeing into the sewers where she gives birth to her son.
The chase intensifies as the government team closes in. In her rage and grief, Sil retaliates violently, resulting in the death of Fitch, while Dan fatally ignites Sil’s offspring, leading to a climactic confrontation. In a final act of defiance, Sil engages Press in a desperate struggle, only to meet her end when he shoots her with a grenade launcher, sending her plummeting into a flaming pool.
In a chilling conclusion, the film hints at the repercussions of Sil's legacy, as a rat consumes one of her severed tentacles, resulting in its mutation—a dark reminder of the horror that emerged from the scientists' hubris.
