Subnautica: Below Zero | Full Game | Longplay Walkthrough No Commentary

Two years after the events of the original game, scientist Robin Ayou smuggles herself onto planet 4546B in a frozen biome named "Sector Zero" in order to investigate the circumstances of her sister Samantha's death, which Alterra claims was a result of "employee negligence". During the past year, Alterra had built a number of research bases on the planet but had recently withdrawn all of its personnel, giving Robin an opportunity to land on the planet.

Shortly after landing, Robin picks up a distress signal and goes to investigate it, discovering an alien sanctuary containing the digitized consciousness of a living precursor. The alien introduces itself as Al-An and forcibly downloads itself into Robin's brain just as the sanctuary's power runs out. Angry at Al-An for intruding into her mind, Robin agrees to help it construct a new body for it to transfer into. In order to carry this out, Robin combs the planet for alien installations and artifacts to find the necessary components and materials. Al-An also explains that it did not try to contact Alterra because it believed Alterra's motivations did not align with its own, and it did not want to risk them finding a way to its homeworld. During their search, Al-An admits that it was one of the lead scientists researching a cure for the Kharaa bacterium and it disobeyed orders by trying to hatch Sea Dragon eggs, leading to the initial outbreak of Kharaa on 4546B.

Meanwhile, Robin also continues her investigation into Samantha's death. She searches all of the abandoned Alterra research facilities, and finds out that Alterra discovered the corpse of a leviathan frozen in ice that was still infected by Kharaa. However, instead of destroying the Kharaa, Alterra decided to start researching the bacterium for useful applications. Fearing another outbreak and concerned Alterra will use Kharaa to develop bioweapons, Samantha decided to sabotage the project by using explosives to collapse the cave holding the leviathan, while her friend, Degasi survivor Marguerit Maida, destroyed the lab holding Alterra's Kharaa samples. However, Samantha was inadvertently caught in the blast and killed by falling rubble. Unable to determine the cause of the cave in, Alterra wrote off Samantha's death as a result of negligence. In order to prevent Alterra returning to the planet to restart their research, Robin recovers a cache of antibacterial agent Samantha had made earlier and uses it to neutralize the Kharaa infecting the frozen leviathan.

Eventually, Robin is able to recover all of the components necessary to build a new body and fabricates it. Al-An transfers itself to the new body and decides to return to the precursor homeworld in order to see for itself what had happened to the other Architects, as well as to atone for its past mistakes. Al-An activates an Architect phase gate and Robin travels with Al-An to the precursor homeworld, uncertain of what they will find there.