A team of scientists working on a secret government project called the Search for Extraterrestrial Intelligence (SETI) receives a transmission containing alien DNA sequences along with instructions on how to splice them with human genes. The lead scientist, Xavier Fitch, decides to conduct the experiment in an attempt to see what the resulting organism would look like. The experiment results in the birth of Sil, a seemingly innocent human female, who is genetically an alien hybrid. As Sil rapidly matures into an adult, despite only being a few months old, it becomes apparent that she possesses dangerous traits inherent to her alien DNA, including incredible strength, agility, and a predatory instinct.

Recognizing the potential threat, the government decides to shut down the project and eliminate Sil. However, Sil escapes containment, proving to be far more cunning and lethal than anticipated. As she ventures into Los Angeles with a drive to mate and propagate her species, the government assembles a team of specialists to track and destroy her. This team includes a mercenary, an empath, a biologist, and an anthropologist. While the empath's ability provides them critical insights into Sil's actions and whereabouts, the team struggles to apprehend her as she blends into the populous city, seeking out a suitable mate to fulfill her biological imperative.