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Overview

The protomolecule is an infectious agent of extrasolar origin that serves as the central catalyst for the events of The Expanse: Osiris Reborn. It is an alien nanotechnology created roughly two billion years ago by an ancient and technologically advanced species sometimes referred to as the Ring Builders. The protomolecule was never intended as a weapon. It is a tool, a self-replicating platform programmed to use biological matter to construct things. When it encounters organic life, it does not simply kill it. It repurposes and transforms it, co-opting biological structures to carry out its programming.

In the game, the protomolecule drives the conspiracy that pulls the player character from a routine shore leave on Eros Station into a solar system-spanning conflict. The player's escape from Eros during the protomolecule outbreak is the inciting event of the entire story, and the alien substance and its effects on human populations remain a threat throughout the campaign.

Origins and Discovery

Approximately two billion years ago, the Ring Builders launched the protomolecule across interstellar space inside an asteroid aimed at Earth's solar system. The goal was to reach a world rich in biomass, where the protomolecule would consume organic material and use it to build a Ring Gate, a stable wormhole connecting the solar system to the Ring Builders' vast interstellar network. The asteroid was captured by Saturn's gravity before it could reach its target. It became Phoebe, one of Saturn's moons, and the protomolecule lay dormant inside for eons.

The Martian Congressional Republic discovered the protomolecule during research operations on Phoebe. Rather than announcing the discovery publicly, the find was funneled to Protogen, a black-ops subsidiary of the megacorporation Mao-Kwikowski Mercantile. Protogen's leadership, particularly executive Vice President Antony Dresden, saw the protomolecule not as a scientific curiosity but as a resource to be weaponized and controlled.

Properties

The protomolecule is a self-replicating nanotechnology that operates at a level of sophistication far beyond human science. Its core behavior is to consume biological matter and repurpose it according to its ancient programming. Key properties include the following.

The Eros Incident

The Eros incident is the event that directly triggers the plot of the game. Protogen orchestrated a controlled infection of the entire population of Eros Station, numbering approximately one and a half million people. The operation began when a ship was destroyed in the station's docks, triggering a station-wide lockdown. The population was herded into what they were told were fallout shelters. These were actually contamination cells, flooded with radiation and the protomolecule. Everyone on the station was infected within hours.

The player character is on Eros during this event, on shore leave from their duties with Pinkwater Security. When the lockdown hits, the player and their twin sibling J are among the very few who manage to escape. The rest of the player's team is killed during the attempt. The survivors commandeer a Protogen vessel and flee, an act that sets the entire game's story in motion.

Julie Mao and the Protomolecule

Julie Mao, daughter of Jules-Pierre Mao (the CEO of Mao-Kwikowski Mercantile), plays a key role in the protomolecule's spread to Eros. Julie was held prisoner aboard the Protogen ship Anubis, which was transporting live protomolecule samples. When the bioweapon contaminated the crew, Julie was the sole survivor. She escaped on a shuttle and made her way to Eros, unaware that she was already infected. She barricaded herself in a flophouse on the station and eventually succumbed to the infection.

Julie's infected body became the nucleus of the protomolecule hive mind on Eros. As the protomolecule consumed the station's population and achieved sentience, Julie's subconscious directed Eros on a collision course toward Earth. Detective Joe Miller, who had been searching for Julie, made contact with the protomolecule intelligence through her and convinced it to divert Eros to Venus instead. Eros crashed into Venus, saving Earth. These events occur in parallel with the game's timeline. The player experiences the Eros incident from the ground level rather than from the perspective of the books' and show's main characters.

Relevance to the Game

The game's narrative takes place during the timeframe of the first two books in The Expanse series (Leviathan Wakes and Caliban's War). This means the protomolecule is at the center of the story from start to finish. After escaping Eros, the player encounters the ongoing fallout of the protomolecule conspiracy as Protogen continues its operations across the solar system. The most dangerous manifestation of this research is Project Caliban, a program to create protomolecule-human hybrid supersoldiers that the player will encounter during the campaign.

The protomolecule also shapes the broader political landscape the player navigates. The three major factions (Earth, Mars, and the Belt) each have different reactions to the protomolecule threat, and the player's faction alignment and choices determine which perspective they experience most closely.

Key Facts