Babylon's Ashes picks up right after the fallout of the previous book. Earth and Mars attempt to recover from their separate problems while joining forces to oppose the Free Navy. In their weakened condition, they will need assistance from the Outer Planets Alliance. Fred Johnson calls a meeting of the remaining OPA leaders who have not sworn their allegiance to Inaros, while Marco keeps close tabs on the movements of the Rocinante. The Free Navy has assumed control of Medina in the "slow zone", and are blocking all traffic through the gates in an attempt to assert Belter supremacy. The Earth-Mars coalition decide their only recourse is to re-take Medina station and reopen the gates. Meanwhile, the rogue faction of the Martian military have established a colony in the Laconia system and posted a "No Trespassing" message.