The Expanse has so many plot details spread throughout its four seasons that it becomes easy to forget some crucial ones. Beyond the machinations of Inaros' Free Navy, which has set the stage for the fifth season of the sci-fi drama, there's also an internal political struggle on Earth and military corruption on Mars. These story threads seem to be very grounded for a space opera, that is until you remember that the show also had, for lack of a better term, space zombies.
As soon as Julie Mao found the reactor core of the Scopuli encrusted with the glowing remnants of the crew, it was clear that The Expanse held horrors within its narrative. One such horror is the protomolecule, which serves as a catalyst for the aforementioned space zombies, as well as carrying with it the implication of extra-terrestrial life from beyond the limits of human expansion. With that said, let's look at everything revealed about the protomolecule so far.
The Protomolecule Feeds on Radiation
In Season 1 of The Expanse, the crew of the newly minted Rocinante and Detective Miller of Ceres Station uncover a conspiracy that has deadly implications for the people of Eros Station. Their independent investigations converge on Eros, where they discover that Julie Mao is dead from some kind of infection, causing her body to erupt as the contaminant attempts to escape its host. It quickly becomes apparent that Julie attempted to destroy all power sources in the room to halt the infection's progress, but her struggle was in vain.
In Season 1, Episode 9, "Critical Mass," Dr. Dresden informs Jules-Pierre Mao, Julie's father, that she has died, but also that they managed to secure a sample of her blood, which contains enough protomolecule for the next phase of their experiment. Despite his grief, Mao gives the go-ahead for the operation on Eros to proceed, leading the station to become a mass-infection site, with the space docks locked down and the people unable to escape. Eros security, former criminals hired by Protogen, inject the civilian population of Eros with the protomolecule and exposed them to large quantities of radiation to accelerate its growth, taking it to its next stage.
The Protomolevule Creates Vomit Zombies
In Season 1, Episode 10, "Leviathan Wakes," the irradiated people of Eros are released from their containers as they enter the second stage of infection -- vomiting the protomolecule onto survivors. These "vomit zombies" become an infection vector, further spreading the protomolecule throughout the station, dooming the population.
Dresden watches through video feeds as the population of Eros succumb to the protomolecule infection spreads throughout the station and commands that the results be sent to Thoth station. What he fails to see, however, is that the protomolecule was becoming something more than just an infection as it moves and attacks people through the use of spores and what seem to be prehensile tendrils that grab unsuspecting victims and pull them towards its mass. Rather horrifically, the protomolecule also has the ability to create a hive mind consisting of the minds it has absorbed, and it projects their voices into the universe using a radio signal.
The Protomolecule Reshapes
Following the controlled infection of Eros, Protogen seeks to further their experiments by integrating the protomolecule with human physiology and create a hybrid being that will serve as living weapons. The project became known as Project Caliban, and its first test saw a Hybrid soundly defeating a group of Earth soldiers, as well as Bobbie Draper's Martian marine squad, outside Ganymede Station. The Hybrid doesn't require a spacesuit when fighting in a vacuum, making it a deadly foe to be dealing with in space.
However, the protomolecule isn't limited to living physiology, as it can also reshape and repurpose whatever it comes into contact with. Following Eros Station's descent into Venus, the protomolecule began to reshape the planet from the inside, erecting structures and creating devices that enabled instantaneous outbound communication. The protomolecule had begun to demonstrate that it had limitless potential, beyond what the Protogen corporation had envisioned. It displayed its power by first instantly deconstructing the Arboghast, leaving its crew out in the vacuum of space, and eventually launching the Ring gate -- a massive structure containing a wormhole which led to the Slow Zone and the Ring Station, allowing access to hundreds of new, previously undiscovered star systems.
The Protomolecule Is an Extrasolar Object
It's later discovered that the protomolecule was actually created by a long-dead alien civilization as a means of terraforming a planetary system. The Investigator, now using Miller's appearance, confirms to Holden that this race of beings launched the protomolecule to the Sol System to develop a transport link there, connecting it to their Ring network. However, as Dresden suggests in Season 2, Episode 1, "Safe," the initial probe was caught up in Saturn's gravity and subsequently became Phoebe -- one of Saturn's moons -- leading to Protogen's discovery of the molecule billions of years later.
As Holden is given a vision from his connection to the protomolecule's hive mind, it shows that the creators of the power were wiped out by an even more advanced race. The protomolecule has shown it has the capacity to break physics, meaning whoever killed off the people who created it must be packing some serious firepower.
An adaptation of the novel series of the same name by James S.A. Corey, The Expanse stars Steven Strait, Cas Anvar, Dominique Tipper, Wes Chatham, Shohreh Aghdashloo and Frankie Adams. Season 5 premieres Dec. 16 on Amazon Prime Video.