The first part of this series deals with the aftermath of the ghastly experiment conducted on Eros which led to everyone on the station dying horribly. Holden and his crew on the Roci- and Miller- attempt to find those responsible for the bio-weapon and how to stop it. It turns out to be an alien protomolecule which was discovered and unleashed by scientists in the pay of wealthy businessman/arms dealer Jules-Pierre Mao (father of Julie) who is attempting to weaponize it for profit. Realizing that, if the protomolecule escapes Eros, it will cause even more catastrophic death and destruction, Holden and the others determine that the asteroid must be destroyed. To this end they form an uneasy alliance with Fred Johnson, the OPA leader based on Tycho Station. He and Miller come up with a plan: they will plant a number of bombs on Eros, then steal the gigantic new ship being built by the Mormons for a deep space expedition and drive it right into Eros, exploding the asteroid. All goes according to plan until, impossibly, the asteroid somehow moves out of the way of the ship, which flies harmlessly by. Worse, having left its orbit, Eros now somehow embarks on a collision course with Earth. This is stopped only by Miller sacrificing himself and the Eros being driven straight into the surface of Venus where it explodes.
Meanwhile on Earth, the sight of a huge asteroid hurling towards them is, naturally enough, viewed as a Mars attack even after disaster is averted, and only the fast talking of Avasarala manages to pull the UN council back from the brink of declaring war. She has by this time figured out that her superior Errinwright has been collaborating with Mao, and she has engaged a bodyguard/confidante- Cotyar- because at least one attempt has been made on her life. Which only goes to show you: never trust anyone in the U.N. It's a nest of vipers.
While this is going on, in space above Ganymede the military ships of both Earth and Mars have received reports from their deployed teams that they've been attacked. Each assuming the other is responsible, armed conflict breaks out and a Martian ship is destroyed. Worse, the debris falls to the surface, taking out at least one of the domes and causing widespread, unrepairable damage and many deaths.