The premise of Subnautica is that the player, Ryley Robinson, is the sole survivor of the Aurora, a spacecraft that crash-lands on an alien oceanic planet designated as 4546B. The player's immediate objectives are to survive by managing their health, hunger, thirst, and oxygen, as well as to craft essential equipment, and explore the vast underwater environment.
However, the deeper premise involves uncovering the mysteries of the planet, which include discovering alien structures and the presence of an ancient precursor race. This race was studying a devastating bacterium called Kharaa, which has now infected the planet and the player. The ultimate goal is to find a way to escape the planet, which means discovering a cure for the Kharaa infection, disabling the planetary quarantine system, and building a rocket to return to Earth

Subnautica: Below Zero's premise revolves around Robin Ayou, a xenobiologist investigating her sister's death on Planet 4546B. She arrives at the planet after the events of the first Subnautica, crash-landing in the frozen Sector Zero and seeking to uncover the truth about her sister's demise at the hands of the Alterra Corporation. This leads her to explore abandoned research stations, encounter mysterious alien artifacts, and grapple with the secrets of the planet's ancient alien civilization, the Architects