In this sequel to OLD MAN'S WAR, humanity's intergalactic struggle to survive takes a potentially catastrophic turn when Charles Boutin, the creator of the BrainPal technology that has given humanity's soldiers their slight military edge, turns traitor and forms an alliance with three alien species particularly hostile to humanity's presence in their galactic backyard. When the facts of this betrayal become known, the Colonial Defense Forces, mankind's frontline defense in its forays out of our solar system, decides to build a clone from Boutin's downloaded consciousness to try to glean what he's up to. The implantation doesn't fully take and the "failure" is given the name Jared Dirac and handed over to the Ghost Brigades, the CDF's special forces. While all CDF soldiers are given new bodies, with enhanced physical characteristics (such as boosted strength, endurance, oxygen-producing artificial blood, and a host of other tweaks to help them survive where an average person would be dead in seconds), the Ghost Brigades are a breed apart. Built from the recombinant DNA of the dead, and given brand new consciousnesses designed to integrate perfectly with their BrainPals, they are as far beyond regular CDF soldiers as the CDF is beyond an unaltered human.
At first, Jared Dirac begins his new "life" as a good Ghost Brigade soldier. He participates in numerous raids that give a glimpse of the terrible threats humanity faces from the many alien races who, all being organic creatures requiring similar planets to thrive, compete for the same colony worlds and employ increasingly horrific weapons against one another. But soon Dirac finds Boutin's memories beginning to bubble up to the surface and interfere with his ability to function as the amoral killing machine he is supposed to be. Jane Sagan, Dirac's commander (assembled herself from DNA taken from the dead wife of John Perry, the main character of OLD MAN'S WAR), deduces the changes in Dirac and, at the high command's insistence, they decide to send him with a squad to Boutin's old research lab to hopefully trigger even more relevant memories. The problem: the lab is now in the hands of the Obin, one of the three races allied against humanity.
The Obin are a species without a higher consciousness. They are like high-functioning animals, and their close ties to Boutin are especially alarming, and baffling.
In short order, Dirac's squad infiltrates the lab, taking numerous losses, but killing Boutin in the process. The threat isn't over, though. They discover that Boutin, with the help of the Obin, have invented a virus that infects the BrainPals via their own quantum wiring, and could potentially wipe out the entire CDF military. Boutin, it appears, did not fully appreciate humanity's tinkering with the dead and generally playing God with manufactured consciousnesses.
Dirac is the only one who can stop it. Using his Boutin-imprinted consciousness, he short circuits the whole system, but sacrifices himself in the process.
Boutin's daughter, Zoe, is also discovered hiding out in the lab (along with two Obin bodyguards) and Jane Sagan, along with John Perry, decide to adopt her. We also learn that the Obin have been granted a semblance of consciousness from Boutin, which was the price of their participation in his schemes. This gift from Boutin elevated him to the status of a god in their eyes. With Boutin dead, this reverence passes to Zoe, and with her influence the Obin sign an alliance with humanity.
The current threat abates, but there are strong implications that the greater threats to humanity in the zero-sum race for viable colonies has only just begun.
Best part of story, including ending:
The continued explication of the CDF soldiers, especially the special forces, which received only minimal attention in the previous book, were particularly good. But overall, the plot was not as tight as in OLD MAN'S WAR, and the larger themes of maneuvering among the many alien races was not as involved as in THE LAST COLONY, the next book in the series.
Best scene in story:
The early scenes where Jared Dirac is first being integrated into the Ghost Brigades, and we learn some of their unique culture and training methods, were by far the most interesting in the book.
Opinion about the main character:
Jared Dirac was usually an effective conduit for Scalzi's examination of free will vs. genetic compulsion, but at times this could get a little forced, and in so doing occasionally reduced Dirac to a thematic construct rather than a real, living character.