1. The Precursors and the Kharaa Bacterium Thousands of years before the game, an advanced alien race known as the Precursors (or Architects) arrived on 4546B. The Outbreak: They were searching for a cure for the Kharaa Bacterium, a plague that had already killed billions across their worlds. The Containment Breach: During their research, a Sea Dragon Leviathan attacked their Disease Research Facility, releasing the virus into the planet's ecosystem. The Quarantine: To prevent the virus from spreading off-planet, the Precursors activated the Quarantine Enforcement Platform—a massive gun that shoots down any ship attempting to land or leave. 2. The Sea Emperor's Secret The Precursors discovered that the Sea Emperor Leviathan produced Enzyme 42, which could neutralize Kharaa. The Failure: They held an adult Sea Emperor in a Primary Containment Facility, but she was too old for her enzyme to be effective. The Symbiosis: The Emperor used Peepers to distribute small amounts of the enzyme through the planet's vent system, keeping the ecosystem from collapsing entirely. 3. Human Arrivals The Degasi: Roughly a decade before the game, the Degasi crashed. Three survivors—Paul Torgal, Bart Torgal, and Marguerit Maida—tried to survive in various habitats but were eventually overcome by the environment and the virus. The Aurora: The player-character, Ryley Robinson, arrives on the Aurora, which was shot down while trying to build a phase gate and secretly look for the Degasi crew. 4. Ryley's Journey and the Cure Ryley eventually makes contact with the Sea Emperor, who telepathically guides him to hatch her eggs. The Cure: The baby Sea Emperors produce pure Enzyme 42, curing Ryley of the infection. The Escape: With the infection gone, Ryley is able to disable the Quarantine Enforcement Platform and build the Neptune Escape Rocket to return home (though he ends up with a massive bill from the Alterra Corporation).