Eight months after a convoy of Belter refugees ran the blockade at the Ring gate, the crew of the Roci is tasked with a mission to investigate the planet where the Belters have settled. Meanwhile Bobbie copes with civilian life on Mars, and Ashford serves as a reluctant enforcer of a treaty with the Inners.

Bobbie has an unsettling encounter when Avasarala visits Mars. Holden and Naomi investigate the remnants of an ancient structure on the surface of Ilus, and Murtry accuses the settlers of sabotaging the RCE lander.

Murtry's actions intensify the conflict between the Belters and the Inners on Ilus, and Dr. Okoye joins Holden and Alex in an effort to deactivate the lightning strikes. Bobbie learns she has created bigger problems for her nephew, and finds herself subjected to blackmail.

Murtry and his RCE crew seize control of the colony, putting Amos at the end of a gun while Naomi and Lucia flee toward the Rocinante. Drummond and Ashford get their hands on an OPA pirate, and Bobbie finds herself looking for a new job.

Tensions on Ilus reach a new high when the Belters take hostages. As the U.S. election draws near Avasarala debates her political challenger, Nancy Gao, and on board the Roci Alex and Naomi face a medical emergency.

The inhabitants of Ilus are forced into a reluctant evacuation after a massive explosion sends a shockwave across the planet's surface. Meanwhile the protomolecule appears to render the orbiting ships' fusion reactors inert, and on Mars Bobbie discovers she is good at her new job.

The survivors of Ilus are trapped inside the ruins beneath an ocean of water, facing dwindling supplies and the outbreak of an alien infection. Naomi and Alex work to keep the Barbapiccola from falling out of orbit, while on Earth Avasarala receives intel on the Belter terrorist.

While Naomi and Alex attempt to execute the plan to tow the Barb out of danger, on the surface of Ilus Holden tries to hold together a group of survivors who are quickly losing hope. Avasarala faces the public fallout of her recent military orders, and Drummer confronts OPA leader Fred Johnson.

Miller leads Holden into the heart of the alien ruins with a plan to eliminate the protomolecule once and for all, while Murtry threatens everything by putting his endgame into motion. Naomi and Alex go to desperate lengths to save Lucia and the Belters on board the Barb.

The crew of the Rocinante regroups after the events on Ilus, with the hope of a new frontier opening for humankind. Ashford closes in on the Belter terrorist, and Avasarala faces a personal crossroads. On Mars, Bobbie discovers her companions' involvement in a larger conspiracy.