Season four begins with the opening of the ring gate, a mysterious portal constructed by an ancient alien civilization. Hundreds of new worlds, known as the “ring worlds” or “planets,” are now accessible. The discovery ignites a rush of human expansion, as Earth, Mars, and the Belt all send science and military missions to claim the most valuable new real estate.
The central storyline follows the crew of the Rocinante, who accept a mission from the Belter leader, Anderson Dawes. They travel to the planet Ilus, the first world being settled by Belter refugees. They are tasked with mediating a volatile situation between the Belters and a Royal Charter Energy science team from Earth, which has a legal claim to the planet. The situation quickly deteriorates as ancient alien technology on Ilus begins to activate, causing catastrophic geological and environmental changes.
On Ilus, James Holden is haunted by visions of the long-dead alien “investigator,” who warns him of the dangers of the planet’s alien machinery. Meanwhile, the crew faces impossible choices as earthquakes, deadly slugs, and an overwhelming infection of alien protomolecule technology threaten everyone on the planet. The struggle becomes a race for survival against the planet itself, forcing enemies to work together.
Back in the Sol system, political tensions escalate. Chrisjen Avasarala, now the UN Secretary-General, faces immense pressure as she tries to control the chaotic land grab through the ring gates, fearing a war that could spill across the stars. Her authority is challenged from within as a rival politician undermines her. On Mars, Bobbie Draper works with an underground group, discovering a shocking truth: with new worlds to colonize, Mars is losing its dream of terraforming its own planet, leading to a societal collapse and the selling of military secrets.
The Belt’s story focuses on the rise of Marco Inaros, a radical Belter leader and the father of Naomi Nagata’s son, Filip. He is plotting a bold and terrifying act of terrorism to assert Belter independence, setting the stage for a devastating conflict. Naomi, struggling with her past, is pulled back into this dangerous world as she learns of Marco’s plans.
The season culminates in a desperate effort on Ilus to shut down the alien reactor before it destroys the entire planet. Holden must make a profound choice about communicating with the alien system, ultimately deactivating the threat but leaving many questions unanswered about the ancient civilization’s demise. Meanwhile, Marco Inaros launches his plan, stealth-coating asteroids and hurling them toward Earth, forever changing the balance of power in the solar system as the season ends with a new and terrifying reality.