After being kidnapped by human terrorist group Sapience, Donovan (Exo) is back among his fellow exos, humans grafted with impenetrable armor by the extraterrestrial zhree. Now, after one hundredplus years of peaceful colonization, the zhree are leaving Earth; they'll take select Hardened humans along, and the rest will fend for themselves on the now-vulnerable planet. This sequel continues a nuanced exploration of colonial relations and equality.

Earth, the outlying colony of an overextended alien galactic empire, falls victim to interstellar raiders in this sequel to Exo (2017).The first tentative steps toward reconciliation between the zhree overlords and the human resistance known as Sapience are thrown off-kilter by the sudden announcement from the Mur Erzen Commonwealth's capital that the zhree will be withdrawing for strategic reasons—leaving Earth vulnerable to the Rii, a nomadic, planet-destroying zhree offshoot. Donovan Reyes, a human soldier who has been Hardened with retractable armor and raised since childhood in the group-oriented zhree culture, is pitched into the middle of increasingly violent events and unwillingly finds himself a go-between, desperately seeking common ground between his own species and the zhree colonizers who, having themselves been born on Earth, are reluctant to leave. As it turns out, Donovan isn't the only member of his closely knit unit who nurses hidden connections with Sapience. All the wrestling with guilty secrets and divided loyalties makes for a talky first half, but readers who weather the angst are rewarded with a string of brutal, intense action scenes. Human diversity is indicated by names rather than physical descriptions.Slow to get going, but ultimately wins through to a hard-fought triumph. (Science fiction. 12-18)