Holden and the crew of the Rocinante fight alongside the Combined Fleet of Earth and Mars to protect the Inner Planets from Marco Inaros and his Free Navy's campaign of death and destruction. Meanwhile, on a distant planet beyond the Rings, a new power rises.

The tensions of war threaten to pull the Rocinante crew apart when they make a startling discovery on an asteroid. Avasarala and Bobbie contend with the devastation on Earth as Marco's rocks continue to rain down. In the Belt and on the run, Drummer and her family must make a heartbreaking decision. On Ceres, Marco and Filip's struggles turn inward.

With an unexpected passenger aboard the Rocinante, Holden and crew undertake a dangerous mission that could shift the balance of power in the war. Drummer decides to make an uneasy alliance. Filip wrestles with his guilt.

Avasarala, with the Combined Fleet of Earth and Mars, takes the war to Marco. Filip and Marco's relationship is tested. The war becomes more perilous and personal for Holden and Naomi when the Rocinante is suddenly forced into a battle.

Holden finds himself at odds with his crew when a controversial decision is uncovered. On Ceres, Avasarala struggles to contain an escalating humanitarian crisis. Drummer's growing faction stage a daring raid with dire stakes. Filip, demoted to repair skiff duty, makes an unexpected discovery.

Paths converge on Ceres when Drummer and her fleet arrive to aid the relief effort. Holden tries to convince Avasarala to release crucial information that might avert catastrophe, but she has her own request of him. As Amos and Bobbie get a little R&R at the station, Naomi has a difficult and long-awaited reunion.

In the thrilling season finale, Inners and Belters fight side by side with the crew of the Rocinante in a last, massive, desperate battle with Marco and his Free Navy, with the fate of the Solar System, the Ring Gates, and of all humanity hanging in the balance.