Subnautica and Below Zero

Subnautica

Roughly 1,000 years prior to the game's events, an alien race known as "the Architects" begins investigation of the ocean planet 4546B for a cure to the Kharaa bacterium, which has devastated their civilization. They discover that the Sea Emperor Leviathan species naturally produces "Enzyme 42" in its digestive tract, which makes the species immune to the bacterium. The last of the species is too old to produce the enzyme in sufficient quantities, and attempts to force its eggs to hatch (to extract the enzyme from her young) are unsuccessful. As the Architects grow desperate for a cure, they attempt to extract the enzyme from a related species, specifically the Sea Dragon Leviathan. This leads them to abduct a Sea Dragon egg for research. Following the abduction, an adult Sea Dragon Leviathan attacks the research facility where the egg is being held in an effort to retrieve it. This attack causes the facility to collapse to the sea floor, resulting in the Kharaa bacterium being released into the ocean. With the situation worsening, and the containment breach inadvertently infecting the ocean with the Kharaa bacterium, the Architects place 4546B under quarantine while forcing the remaining Architects to upload their consciousnesses onto cache data hubs.
Centuries later, the spaceship Degasi crashes on 4546B while scanning it for minerals. Three of its crew members survive for a few months on the planet. 10 years later, the Alterra Corporation's spaceship Aurora crash-lands on 4546B while attempting to locate the wreck of Degasi. The player character Ryley Robinson, who is the Non-Essential Systems Maintenance Chief, is among those of the Aurora crew to eject in escape pods. He is knocked unconscious during atmospheric entry, and finds himself to be the sole survivor by the time he wakes up. He is contacted by the trading ship Sunbeam, responding to the Aurora's SOS. When the Sunbeam attempts to rescue Ryley, it is shot down. Ryley investigates and discovers that the weapon responsible is the Architects' Quarantine Enforcement Platform, and that it will continue to shoot down any ship it detects attempting to enter or exit the planet's atmosphere, to prevent the spread of Kharaa. Ryley attempts to disable the weapon; the control panel detects he is infected with Kharaa and, as such, denies his input.
Ryley ventures deeper into 4546B, learning of the Architects' history through exploration of their facilities. He discovers the last living Sea Emperor, still captive in an Architect containment facility, who can communicate telepathically. Ryley activates a teleporter inside her aquarium to allow the Leviathan's young to leave; in exchange, she provides Ryley with the recipe for an enzyme that will allow her eggs to hatch. Ryley uses the enzyme to hatch the eggs; the Leviathan young release large quantities of potent Enzyme 42 into the environment, curing Ryley and halting the bacterium's spread throughout 4546B. Finally at peace, the ancient Sea Emperor bids Ryley farewell and lays down to die.
A cured Ryley can disable the Quarantine Enforcement Platform, construct an escape rocket based on blueprints recovered from the Aurora wreck, and leave the planet. In its final telepathic communication, the Sea Emperor poetically compares itself with Ryley.

Subnautica: Below Zero

Two years after the events of the original game, the Alterra corporation that originally crashed the Aurora on planet 4546B had built several bases on the planet for research on the Kharaa bacterium, and on the alien precursor race, dubbed Architects, that had studied the bacterium prior. Xenologist Robin Ayou smuggles herself onto the planet in a frozen sector named "Sector Zero" to investigate the circumstances of her sister Sam's death, which Alterra claims was a result of "employee negligence." During the past year, Alterra had recently withdrawn all of its personnel, allowing Robin 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 Architect. Desperate for a new storage medium as the sanctuary's power is running out, and after an apparent misunderstanding, the Architect downloads itself into Robin's brain. Angry at it for intruding into her mind, Robin initially tries to ignore it, but soon agrees to help it, now introduced as Al-An, to construct a new body to transfer into. To do so, Robin combs the sector for alien installations and artifacts to find the necessary components and materials. Al-An explains that it did not try to contact Alterra for help before because it believed Alterra's motivations did not align with its own, and did not want to risk them finding a way to its homeworld. During their search, Al-An hints that it was involved with the outbreak of Kharaa.
Meanwhile, Robin also continues her investigation into Sam's death. She encounters Degasi survivor Marguerit Maida several times, who is suspicious of Robin at first thinking she's with Alterra, but reveals her part of the story in return for disabling Alterra surveillance of the sector. Robin searches the abandoned Alterra facilities, and finds out that Alterra had discovered the corpse of a leviathan frozen in ice that was still infected by Kharaa. However, instead of destroying the Kharaa, Alterra decided to research the bacterium for useful applications. Fearing another outbreak and concerned that Alterra would use the Kharaa to develop bioweapons, Sam decided to sabotage the project. Encouraged and helped by Marguerit, she synthesized and stashed away a cure for the Kharaa, then used explosives to collapse the cave holding the leviathan, while Marguerit destroyed the lab holding Alterra's Kharaa samples. However, Sam was inadvertently caught in the blast and killed by falling rubble along with another person. Unable to determine the cause of the cave-in, Alterra wrote off Samantha's death as a result of negligence. Using this knowledge and some maps and clues, Robin recovers the cache of Kharaa antidote that Sam had made and uses it to neutralize the Kharaa infecting the frozen leviathan, finishing her sister's business and preventing Alterra from continuing their project.
Alongside this, Robin is eventually able to recover all of the components necessary to build a new body for Al-An, and fabricates it at an alien facility. After transferring itself to the body, Al-An reveals that it was one of the lead scientists researching a cure for the Kharaa bacterium; it disobeyed orders by trying to hatch Sea Dragon leviathan eggs, resulting in the research facility being destroyed by the Sea Dragon egg's mother and the Kharaa being released onto 4546B. Al-An decides to return to its homeworld in order to see for itself what had happened to the other Architects, as well as to atone for its past mistakes. It activates an Architect phasegate and Robin agrees to leave 4546B with it, uncertain of what they will find there.