A meteor-like object hurtles toward Earth and crashes in the New Mexico desert. A nearby hunter investigates and is attacked by a monstrous alien creature, while his dog flees.
Renee is a 15-year-old girl who lives in New Mexico with her mother Linda, who has sunk into depression after her husband's death, and her selfish, violent new boyfriend Chase. She is an antisocial outsider keeping a selection of reptiles and arachnids as pets. After her distaste for Chase causes an argument at dinner, Renee runs into the desert and comes upon the impact site, where she encounters the alien entangled with the hunter's car and frees it before fleeing. The next day, Renee collects equipment, including Chase's revolver, and returns to investigate. After finding the alien and freeing it from the hunter's jaw trap still attached to it, they form a bond; but after returning home, Renee discovers that she forgot the gun in the desert. Intending to recover it, she finds the creature right outside her house. It creates an empathic link with her, and after recovering from the resulting shock, Renee hides the alien in the basement of her house and names it Croak.
Soon after, Gil, a classmate from school who is interested in Renee, comes by to fetch his video camera, which Renee had borrowed for her alien search, and discovers a recording of Croak on it. Confronted, Renee shows him the alien and extracts a promise from him to keep quiet about it; however, CIA Keyes and his partner Browne have been investigating Croak's arrival, followed its trail back to Renee and overheard her and Gil's conversation. The next evening, Chase confronts her about his missing gun and becomes violent to her. Alerted by her distress, Croak rams the basement's wall, shaking the house and breaking up the fight, and Linda throws Chase out of the house.
The morning after, Renee is picked up by Keyes, who tries to coax her into confessing about Croak, which Renee resists. Linda, believing the shaking was due to an earthquake, calls a plumber to check the water pipes in the basement. The plumber finds Croak, flees in a panic and is killed in a car crash. Upon discovering this, Renee takes Croak into the desert at night to meet with Gil, who tells her that he was visited by Browne, who threatened to deport his grandfather if he didn't tell him about Croak. Renee decides to run away, but is caught by Chase, who prepares to kill her for turning Linda against him. Sensing her distress, Croak arrives and kills Chase before running back into the desert after seeing Renee's fear.
Right afterwards, Keyes arrives and restrains Renee; the CIA sets up a trap, and Keyes brutally breaks Renee's finger to lure Croak to them. Croak is captured as it comes to Renee's aid, and they are taken to the local sheriff's office. Keyes tells Renee that he suspects Croak being a creature bred to act as a scout for a planned alien invasion and intimidates her into cooperating with him on a research of their empathic bond. Briefly left alone after a face-to-face with her mother, Renee deliberately breaks her finger again, agitating Croak into escaping captivity and freeing Renee. Keyes catches up with Renee and Croak, and shoots at them with lethal intent. To protect Renee, Croak breaks cover and rushes Keyes, killing him before collapsing, mortally wounded. The film ends with Renee mourning Croak's demise, while the alien's corpse is studied by CIA forensics until it unexpectedly revives.
