A six-member crew aboard the International Space Station is on the verge of one of humanity’s most significant discoveries: evidence of extraterrestrial life on Mars. The organism, named Calvin by Earth's children, is initially studied by exobiologist Hugh Derry, who finds it to be a multicellular life form, displaying remarkable intelligence and agility. As the crew conducts various experiments, Calvin quickly evolves, becoming larger and more complex. Excitement over the discovery soon turns to dread as the life form demonstrates dangerous characteristics.

The situation aboard the ISS escalates when an accident in the lab causes Calvin to enter a dormant state, leading the crew to erroneously conclude it might be dead. In an attempt to revive it, Calvin is subjected to an electric shock. The organism displays survival instincts and a capacity for aggression that catches the crew off guard. As Calvin grows stronger and more hostile, the team's focus shifts from scientific discovery to self-preservation. The crew must work together to contain the creature that proves to be smarter and more resourceful than anyone anticipated, leading them to confront moral, ethical, and practical dilemmas in their efforts to prevent it from reaching Earth.