The crew of the Roci investigates a strange protomolecule ruin on an alien planet. Bobbie copes with civilian life on Mars. Drummer and Ashford battle with piracy in the Belt.

New alien threats emerge on Ilus as Naomi battles health challenges on the planet. Bobbie searches for a missing person on Mars. Avasarala faces a new challenge in the UN.

As tensions on Ilus reach a boiling point, Holden reveals a secret. Avasarala debates Nancy Gao while an emergency on the Roci forces Alex and Naomi into action.

Avasarala faces fallout for the UN's military mission. Ashford and Drummer makes big decisions about their future within the OPA. A spreading illness on Ilus takes a toll on Amos. Alex and Naomi's attempt to rescue the Barb turns desperate.

Ashford closes in on an enemy while uncovering a dangerous plot. Avasarala faces a crossroads in her personal and political life. The Roci crew works to reestablish life on Ilus in the wake of a deadly catastrophe.

Holden and Fred deal with infiltrators on Tycho. Drummer's past comes back to haunt her. Amos returns to Baltimore. Alex and Bobbie's investigation on Mars leads to rogue soldiers.

Marco's grand plan shocks Earth, Mars and the Belt.