Commander Patrick Ross leads a crewed mission to Mars. Soil samples collected by the astronauts unwittingly contain an alien-based substance which thaws aboard their capsule due to the change in temperature and contaminates them, causing a seven-minute contact gap with mission control. With seemingly no subsequent negative effects, the three astronauts return to Earth to public celebration. Only an institutionalized former scientist, Dr. Cromwell, reacts to their return with violent panic. Meanwhile, Dr. Laura Baker has created a clone of Sil named Eve, whose alien DNA is suppressed to make her more docile. Her team conducts experiments on Eve, hoping to find a way to combat the alien species should it ever return to Earth. Every experiment is unsuccessful as Eve's biology adapts.Upon their return, the team members are told to refrain from sexual activity for ten days. Patrick disregards this advice and has a threesome with two sisters Marcy and Lucy following a fundraiser. During sex with Lucy, he begins to sprout tentacles. Lucy notices his transformation while she is stroking his body and is horrified at the sight of him, whereupon she tries to free herself from his clutches. She pleads with him to stop but he ignores her pleas as he proceeds to climax. The hyper-fertile alien seed within Marcy and Lucy causes them both to experience accelerated pregnancies, culminating when Patrick's half-alien children violently burst from their abdomens. Patrick hides his rapidly growing sons on the property of his father, Senator Judson Ross. Each time Patrick has sex, Eve experiences excitement. The next day, Patrick tells his father he cannot remember the previous night's events. Senator Ross indicates that he is aware of Patrick's behavior and tells him to focus on his political goals.Dr. Orinsky, one of the NASA scientists who examined Patrick, discovers something amiss about his blood sample and desperately tries to contact Cromwell. However, Orinsky is ambushed and disemboweled by something formed from Patrick's blood. Laura discovers that the DNA in Orinsky's wounds is distinct from Eve's, making her an unlikely culprit in the murder. This prompts Colonel Burgess, the military supervisor in charge of her project, to reunite her with Press Lennox to contain the new threat. Press and Laura seek out Cromwell, Orinsky's former professor, and learn that he discovered that the alien species had attacked and destroyed Mars in ancient times. Because of his fears that alien DNA might remain on Mars to infect anyone who visited the planet in the future, Cromwell urged the government to abort the mission, but was committed to an institution to permanently silence him.Press and Laura identify the Mars astronauts as the prime suspects. Unable to find Patrick, they pursue the other astronauts, Anne Sampas and Dennis Gamble. They arrive too late to prevent Anne from having sex with her husband, resulting in her impregnation with hybrid offspring. A tentacle-like creature bursts from Anne's abdomen and kills her husband before Press and Laura manage to kill the monster. Meanwhile, a second team of agents makes it in time to locate and examine Dennis, and confirm he is not infected. Laura discovers that Anne's hybrid DNA does not match that in Orinsky's wounds, meaning Patrick is the killer. Meanwhile, Patrick has sex with his girlfriend, resulting in her death through the birth of another hybrid son named Portus. Horrified at causing her death, Patrick attempts suicide by shotgun, but the alien DNA regenerates his head and restores him back to life and temporarily killing his human side, with the drive to mate with as many women as possible. Dennis witnesses this and tells Press and Laura what he saw, remembering what happened in the capsule and joining them in their mission. Patrick begins impregnating as many women as he can, hiding his victims and up to twenty children on his father's property.Laura is ordered to activate Eve's alien DNA so she can telepathically track Patrick, but this makes her volatile and strengthens her alien instincts. Patrick surrenders to Press and Dennis after attempting to rape Darlene, whereupon he becomes aware of Eve; they take him to the lab despite their suspicion. As Patrick is taken to custody, Eve shows signs of being in heat. Patrick demands Laura open Eve's cell and nearly kills her when she refuses, but Press and Dennis drive him off with a special gaseous weapon. Burgess confronts Ross with irrefutable proof of Patrick's infection and demands help in detaining Patrick. Suspicious that Patrick will instead be killed, Ross deduces that he is at the property and apologizes for treating his son so coldly. Patrick's human side briefly returns, but while he tearfully embraces his father, the alien DNA suddenly violently reasserts itself and kills the Senator. Ross's death completely destroys Patrick's humanity, and he helps his hybrid children to cocoon, so they will soon begin mating themselves and prepare to eradicate humanity.Back at the lab, Laura discovers Dennis was not infected because he is a carrier of sickle-cell disease, as the species lacks immunity to human genetic diseases. While they plan to weaponize this weakness, Eve breaks out of her confinement to find Patrick. Press, Laura and Dennis pursue her, with Burgess and the military following. At the shed, they kill Patrick's offspring while Eve and Patrick transform into their alien forms and begin to mate. The mating stops when Press and Dennis intervene, allowing Eve to break free; Patrick, transformed into another alien form, apparently attacks Eve by forcing his tongue-tentacle down her throat. Patrick attempts to attack the team, but Press impales the male alien in the back with a pitchfork coated in Dennis' blood, causing him to disintegrate and die.The military escorts Press, Laura and the injured Dennis away. Eve's seemingly lifeless body is loaded into an ambulance, but shortly after, her womb begins to swell rapidly-indicating her survival and impregnation by Patrick - while Portus, who somehow survived, watches. As the screen cuts to black, Eve's uterus bursts and her screaming is heard.