The six-member crew of the International Space Station is tasked with studying a sample from Mars that may be the first proof of extra-terrestrial life, which proves more intelligent than ever expected.
The six-member International Space Station (ISS) crew capture a probe returning from Mars with a soil sample that might contain evidence of extraterrestrial life. Exobiologist Hugh Derry revives a dormant cell from the sample, and it quickly grows into a multi-celled organism that American school children name “Calvin”. After an atmospheric accident in the lab, Calvin becomes dormant. Hugh revives Calvin with mild electric shocks, but Calvin immediately becomes hostile and attacks Hugh, crushing his hand. While Hugh lies seemingly comatose from Calvin’s attack, Calvin uses the electric shock tool Hugh wielded to escape his immediate enclosure. Now free in the lab room, Calvin devours a lab rat and grows in size. Engineer Rory Adams uses the opportunity to enter the room and rescue Hugh. However, Calvin latches onto Rory’s leg and physician David Jordan locks Rory in the room to keep Calvin contained. After Rory unsuccessfully attacks Calvin with a flame thrower, Calvin enters his mouth, killing him from the inside. Emerging from Rory’s mouth even larger, Calvin escapes through a vent. Hugh theorizes that lack of breathable air on Mars is what kept the organism dormant.
Finding their communication with Earth cut off, mission commander Katerina Golovkina performs a space walk to fix the antenna. Calvin attacks her outside the ISS, rupturing her spacesuit’s coolant system in the process, causing liquid to fill her helmet. She refuses to open the airlock to seek help, keeping Calvin out of the station but causing her to drown in her spacesuit.
Calvin attempts to enter the station through the thrusters. The crew try to use the thrusters to launch Calvin into deep space, but their attempt fails and the station loses too much fuel. The ISS enters a decaying orbit, which will eventually cause the station to burn up in Earth’s atmosphere. Pilot Sho Murakami informs the crew that they need to use the remaining fuel to get back into a safe orbit, but the attempt would allow Calvin back into the station. The crew plan to make Calvin dormant by sealing themselves into one module and venting the atmosphere from the rest of the station.
When Hugh enters cardiac arrest, the crew realize that Calvin had attached itself to Hugh’s leg and was feeding off of him. Having grown into a larger tentacled creature, Calvin attacks the remainder of the crew. Sho seals himself inside a sleeping pod as Calvin attempts to crack the glass and get to him. David and quarantine officer Miranda North use Hugh’s corpse as bait to lure Calvin away from Sho and trap it in a module to deprive it of oxygen…