Inside a traveling spacecraft, a robot pilot named SAM-104 is at the controls as he approaches Mike Colby, who is encapsulated, unconscious, in a glass chamber resembling hibernation stasis. Once Mike awakens, the robot alerts him that their ship is under siege. With quick reflexes, Mike teams up with SAM-104 to fend off the attackers by firing missiles. Shortly thereafter, the robot informs him that the Federation has extended Mike's mission, redirecting their course to the planet Xarbia for the purpose of repairing a security lab. They leap into hyperspace, arriving at the surface of Xarbia, a desolate planet illuminated by two orbiting suns.
Upon arrival, Mike meets Dr. Gordon Hauser and Dr. Barbara Glaser. He proceeds to disable SAM-104's controls, allowing for free engagement. The doctors reveal their work in bacteriological genetic engineering and the successful creation of a new food source. As Mike encounters crew member Tracy Baxter, Gordon instructs her to locate their missing colleague, Jimmy Swift, and escort him to the lab. Within this lab, Mike meets Dr. Cal Tinburgen, the head of bacteriology, and is taken aback by the disarray of bloody rabbit carcasses littering the floor. The scientists explain that the chaos was caused by an octopus-like creature referred to as "Subject 20," which possesses the unique ability to constantly alter its genetic structure. Mike proposes that they should eliminate the creature, but Barbara persuades him to hold off for the moment.
As Tracy joins Mike and the others, including crew members Earl Richards and Brian Beale, for dinner, Jimmy, who is busy cleaning the lab, notes an increase in the creature's breathing. Gordon reassures Jimmy over the intercom that this is expected behavior but still sends Tracy back to check on the situation. Suddenly, a slimy, black blob surges forth from Subject 20 and assaults Jimmy. Meanwhile, the doctors elaborate on their experiments involving a genetically spliced substance known as Proto-B combined with rapid-growing algae. When Mike inquires about what was spliced with Proto-B to bring about Subject 20, Barbara cryptically refers to a deceased crew member named Annie but offers no further details. A distress signal soon interrupts their conversation: Tracy has discovered Jimmy’s body in the lab, his brain missing yet his heart still beating. As Cal carries the corpse into the medical bay, Barbara expresses her sympathies to Tracy.
That evening, Earl observes the security footage that captures Barbara and Mike in a passionate encounter before spotting an alarm alert. He descends into a dark corridor, only to find himself confronted by the creature. Meanwhile, Cal administers medication to himself while witnessing the horrific transformation of Jimmy's skin and muscles into a gelatinous substance.
The following morning, Mike unexpectedly finds Tracy in the shower, completely unguarded. Just as he disrobes, the creature strikes, but they narrowly escape. The monster crashes through the sauna skylight, prompting Mike to declare that he, Gordon, Brian, and SAM-104 will pursue it outside. The team dons protective spacesuits as Gordon warns them of the impending atmospheric danger that will cause nitrogen in their blood to froth. Back in the lab, Barbara and Cal observe as the gelatinous mass splits into two.
On the arid surface of Xarbia, Brian stumbles upon Earl's decomposing body near the mutant's recent cocoon. As the men fire laser shots at the sac, they discover it has been vacated. Near their station, they encounter the creature, now a massive, tentacled beast with an ominous, gaping mouth. To their horror, the mutant devours Gordon in one swift motion before disappearing down a vent, wreaking havoc in the control room and obliterating all computer systems. Cal realizes that the creature plans to create a perpetual food supply by injecting Proto-B into the bodies of its victims, facilitating their continuous reproduction. Gordon stumbles in, severely disfigured and on the verge of collapse. Cal conveys the grim truth that Gordon’s cells were spliced with Proto-B and used to fertilize an egg implanted into Annie, which led to Subject 20’s rapid growth and ultimately resulted in Annie's demise.
Meanwhile, in the showers, Barbara confides in Tracy that they should strive to communicate with the creature rather than attack it. As Mike and the other men brainstorm methods to eliminate the mutant, the two women venture into the control room. Recognizing that the creature is capable of understanding her, Barbara types on the keyboard to inquire about a potential coexistence with humans, only to be savagely stabbed by the creature's tentacle. Tracy races to inform the men of the tragic event, and Mike along with Brian quickly follow her back to the control room. SAM-104 discloses that it is aware of Cal’s cancerous condition; in a desperate act, Cal injects his cancerous cells into a sample of Gordon's mutating tissue. Subsequently, he detonates the remaining food supply of the creature, preparing to join Brian and Mike in the control room.
In a shocking turn, the creature’s cocoon releases a liquid that electrocutes Brian. As Cal attempts to exterminate the creature, he falls victim to a brutal blow from a tentacle. Accompanied by SAM-104, Mike heeds Cal's orders to transport him to another lab where Cal instructs Mike to excise his liver tumor and offer it to the monster as sustenance. In a race against time, Tracy dashes to retrieve Cal's morphine supply, but fate takes a dire turn when she is pursued by the monster through the halls. Faced with urgency, Mike performs the crude surgery to remove Cal's malignant tumor without waiting for the morphine. As he completes the procedure, Tracy, having managed to evade the beast, returns just in time to administer the morphine to Cal—only for him to succumb immediately afterward.
Suddenly, Subject 20 bursts into the lab, instigating a frantic defense. Mike tries to get the creature to consume the tumor while SAM-104 unleashes its ray gun in a futile effort. In a valiant act, SAM-104 takes a scalpel blade and stabs the monster in its right eye, but the creature retaliates with a fierce blow, shattering the robot into multiple pieces. In a climactic moment, Mike succeeds in offering the tumor to the mutant. In an intense response, the monster violently expels the tumor and collapses, dying a slow and agonizing death. Surrounded by the remnants of their devastated crew and in the midst of carnage, Mike and Tracy reflect on the cost of survival, the gravity of their ordeal lingering in the atmosphere long after the horrific events have unfolded.
