A six-member crew aboard the International Space Station performs a successful capture of a space probe returning from Mars with a soil sample. The crew is tasked with studying the sample, which could prove to be the first evidence of extraterrestrial life. After extracting a single cell from the sample, British biologist Hugh Derry, an expert in exobiology, is able to revive the dormant organism with atmospheric adjustments and glucose. It quickly grows into a multi-celled organism and reacts to stimuli. The crew discovers that each cell in the organism is a myocyte, neuron and photoreceptor, all at the same time. The Quarantine Officer Dr. Miranda North remarks that the organism is "...all muscle, all brain and all eye."

The organism, now named Calvin by school children in America, evolves and grows quickly. After an atmospheric accident in the lab, Calvin becomes dormant. Concerned with its survival, Hugh attempts to revive Calvin with a mild electric shock. Calvin re-animates, becomes hostile and grabs hold of Hugh's right hand. Hugh struggles to get free as Calvin crushes his hand causing Hugh to pass out from the pain. Calvin escapes the containment cube into the room with Hugh.

While it devours a lab rat and immediately evolves and grows in size, American engineer Rory Adams enters the room and rescues Hugh. However, sensing his presence, Calvin latches onto Rory's leg. Crew Doctor, David Jordan, locks Rory inside the room to keep Calvin in containment. Rory manages to remove Calvin from his leg and attacks it with a handheld incinerator. Calvin shows remarkable resilience to fire and evades Rory's attacks. Calvin then attacks Rory and forces its way into his mouth, killing him from the inside. Emerging from Rory's mouth, evolved and even larger, Calvin then manages to escape from the room through an emergency fire suppression vent. Hugh theorizes that the lack of breathable air on Mars is probably what kept the organism dormant until that point.

Finding their communication with Earth cut off, mission commander Katerina Golovkina performs a space walk to fix the antenna. Calvin has consumed ISS coolant as a source of nutrition, and now escapes through a valve on the antenna and attacks Katerina outside the ISS. The attack ruptures Katerina's EVA suit's coolant system, effectively causing drops of water to block her vision. Soon her helmet becomes filled with water, drowning her inside the suit. Katerina sacrifices herself by refusing to open the airlock back into the shuttle in an attempt to keep Calvin out.

Calvin attempts to re-enter the station through the thrusters. The crew then decided to use the thrusters to launch Calvin into deep space. However, this fails and causes the crew to lose too much fuel. An attempt to stop Calvin results in the ISS entering a decaying orbit, which would cause the ISS to fall to the Earth's atmosphere. Japanese pilot Sho Murakami informs the crew that they need to use the remaining fuel to get back into a standard orbit, but that it would allow Calvin back into the station. The crew formulate a plan to try and make Calvin dormant by sealing themselves into one module and venting the atmosphere from the rest of the station.

When Hugh suddenly enters cardiac arrest, the crew manages to resuscitate him only to realize that Calvin had attached himself to Hugh's leg (who is paraplegic) and was feeding off of him. Now having grown into a much larger tentacled creature, Calvin attacks the remainder of the crew. Sho makes a run for the sleeping cabin and seals himself inside his pod as Calvin attempts to crack the glass and get to him. After Hugh dies, Miranda and David realize that Calvin has ingested one of the vital sign trackers on Hugh's suit and begin to track him. Using Hugh's body and blood as bait, they lure Calvin away from Sho and trap it in a module to deprive it of oxygen.

Having received the SOS call just prior to the damage to the ISS communication system, Earth Control sends a Soyuz capsule as a fail-safe plan to push the station into deep space, later revealed to be a protocol pre-designed by Miranda as a final firewall to prevent any possible danger. The capsule docks against the module where Calvin was contained and starts pushing the station into deep space. Presuming it to be a rescue attempt, Sho leaves his pod and rushes to board the arriving ship, attempting to manually force open the hatch to enter the capsule. As soon as he opens the hatch, Calvin attacks him and the rest of the Soyuz crew. The crew tries to save Sho, but Sho's actions cause a docking breach that results in the detachment of the capsule, which then spirals back and crashes into the ISS.

The incident causes the temperature and oxygen levels in the ISS to fall rapidly. David and Miranda, the only survivors, realize that the incident has also caused them to re-enter a decaying orbit that will culminate in re-entry into Earth's atmosphere. Aware that Calvin could possibly survive re-entry, David recalls the two emergency escape pods that are programmed to auto-pilot towards Earth and plans to lure Calvin into one of the pods before manually overriding the system to fly into deep space, whilst Miranda could escape in the other pod to return to Earth.

David leads Calvin into his pod and launches into space as Miranda launches hers. Her pod hits debris from the ISS and is knocked off course. Calvin attacks David as he struggles to send his pod into deep space. The two pods then divert. The earthbound pod performs a controlled re-entry and lands in a body of water near two Vietnamese fishermen. As they approach the pod, it is revealed to be David's, who is encased in a web-like substance. Meanwhile, Miranda's navigation system fails and her pod is hurled into deep space. David, still alive, tries to warn the fishermen not to attempt a rescue. The fishermen open the pod door as more fishing boats arrive.