IT'S ALIVE III: ISLAND OF THE ALIVE

Several years after the first two films' events, a woman goes into labor in a cab on a rainy night. Panicked, the cab driver seeks out a police officer to assist in the birth before searching for a public phone to call an ambulance. While he's away, the woman gives birth to a mutant baby. Recognizing it as a mutant child like those from the prior films, the officer tries to shoot and kill the infant, who reacts by killing the officer and mother. The following day, the mutant baby's corpse is found inside a Catholic church, where it dragged itself to die.

In a courtroom, Stephen Jarvis is pleading for the court to spare his mutant son's life, who he argues acts aggressively because it's reacting to the hostility of the people and chaos surrounding him. The baby breaks out of its cage, but Jarvis calms it, convincing the judge to spare the child and four others like him by quarantining them on a remote deserted island. After the trial, Jarvis is a social pariah, unable to work his former acting job and the child's mother, Ellen, wants to live her own life without him as if she never gave birth. Jarvis soon becomes remarkably bitter, as he can't pay his legal fees and women want nothing to do with him, afraid that he'll pass on the mutation through casual touch. Aware that the babies are still alive and the mutations were a side effect of a medication his pharmaceutical company produced, Cabot and some of his associates travel to the island. They hope to kill the babies to manufacture the drug under a new label, only for the mutant babies to kill and eat the entire party.

Five years later, Lt. Perkins approaches Jarvis, telling him that Dr. Swenson has recruited him to launch an expedition to the island to study the babies' growth and wants Jarvis to accompany them. The trip proves to be disastrous; only Jarvis and Perkins survive - Perkins has been deserted on the island while Jarvis remains on the boat as the mutants' captive as they want to travel to Cape Vale, Florida. While traveling, Jarvis realizes that the babies grew quickly and have reached adulthood, as one of them has given birth to a baby, with the father implied to be Jarvis's son. Jarvis discerns that the mutants communicate with each other telepathically. He also determines that the only reason he's still alive is because of the existence of the ship's sailing crew's bodies and because his son has been protecting him. He also realizes that the children are traveling to find Ellen. Eventually, their ship comes across another vessel, at which point Jarvis's son throws his father overboard to save his life, expecting that the ship will pick him up.

When Jarvis awakens, he finds himself held captive in Cuba but manages to convince his captors of his identity, that the mutant children pose a danger to those around them, and to get them to take him back home. Meanwhile, the children arrive in the United States, where they promptly kill several people they consider a threat to either them or Ellen while also defending a woman being attacked by killing a gang of punks. Ultimately both Jarvis and the mutants find Ellen, upon which point the mutants try to get her to take the child. Initially reluctant, Ellen accepts the child after Jarvis convinces that the mutants looked for her out of love for their child, as they're dying of measles and would be unable to care for the child and because they strongly instinctually associated her with motherhood. The two accept the child just as its parents die from measles, while the final remaining adult mutant distracts the police, allowing Jarvis and Ellen to escape. The film ends with the two driving away together with the child, searching for a safe place to raise it.

Jarvis's son, now fully grown, sacrifices himself to save Jarvis, ensuring his father's survival as the mutants seek out Ellen. The mutant lineage continues, as one of the adults gives birth to a new mutant baby.

The ending reflects on the themes of parental love, outcast survival, and the cycle of mutation. Jarvis's journey from public pariah to reluctant protector is completed by his son's selfless act, suggesting a complex, tragic bond between human and mutant. The birth of a new mutant child leaves the door open for ongoing consequences, underlining the film's meditation on responsibility and the unintended results of human interference.