Summary: Bruce Maddox (who originally insisted Data was only a machine in TNG: The Measure of a Man, but now is an AI advocate and even argued for B-4's sentience), has a lab at the Daystrom Institute where he is studying all remaining Soong-type androids (B-4, Lal, a brainless-Lore, and some earlier prototypes). In Nemesis, Data uploaded all his memories to B-4, but since B-4 is a much less advanced prototype, B-4's neural net is in danger of cascade failure due to information overload. Maddox calls the Enterprise for help saving B-4, but moments before Geordi beams down, the lab is attacked by an unknown elite group of infiltrators, who steal all the androids.
The Enterprise sends security teams to the planet to investigate. When a facial recognition camera spots a very Data-looking android, the teams try to intercept but are out-maneuvered at every turn and the android escapes. Back on the Enterprise, they've set up a system of probes, presuming a cloaked ship will be how the androids are transported away. Their plan is initially a success, but the Enterprise can't get a good trace on the cloak--luckily, a small ship, piloted by the android from the planet, takes up pursuit of the cloaked ship (they hope).
Enterprise discretely follows both ships to a desolate planet in Breen space. Because of the Typhon Pact, the Enterprise can't be seen in this system, so the ship hides in the atmosphere of a lightning planet nearby. Picard sends an away team (Worf, LaForge, Choudhury, et. al.) on a covert mission to the surface by launching them in torpedo tubes that burn up in the planet's atmosphere, leaving the passengers inside to parachute drop to the surface. On the planet, they find the android they were tracking, who turns out to be Noonien Soong himself.
Flashback to Soong's life during the events of TNG: After his apparent death in TNG: Brothers, it turns out he actually faked his death and uploaded his mind into his most advanced android yet--just like he did with his wife, Juliana in TNG: Inheritence, but without the built-in self-expiration. His immediate goal was to find Juliana and fix her programming so she won't expire, but she dies before he has a chance. Soong is torn with grief, but decides to devote himself to finding a way to return her to life. However, an old coleague of his, Emil Vaslovik (Flint from TOS, also Cochrane's buddy from the book "Federation") manages to "resurrect" Juliana first--then elopes with her to a distant planet.
Soong, knowing he's lost Juliana forever, sells all his assets, buys himself a moon, and builds a super-advanced lab--turning his efforts towards saving B-4 and bringing Data back to life (Soong hacks into Starfleet transmissions and reads Data's logs to keep tabs on him, and was furious when he found that Data had sacrificed himself to save Picard). Just when Soong has everything about figured out and has built his most advanced android body yet, the Borg launch their invasion into Federation space (Destiny) and destroy his entire planet (they don't even know he's there). Aware that B-4 doesn't have much time left, and with few options remaining, Soong decides to infiltrate the Daystrom Institute, planning to use the computers there to upgrade B-4's programming and save him--but the Breen infiltrate just moments before and steal all the androids.
Present on the Breen planet: The away team discovers an enormous factory, originally built by the dissident Borg from TNG: Descent, but taken over by the Breen (assumedly these Borg were affected by "Destiny" as well). The factory is pumping out millions of copies of Lore, but they lack the complete programming to actually function. The away team and Soong are ambushed by elite Breen soldiers in cloaked suits, and Choudhury (and another red shirt) are vaporized. Soong barters for their lives by saying he can get the androids working, with LaForge's help. Soong does exactly this--and commands all the androids to attack the Breen on sight. With little time remaining before B-4's brain melts, Soong sacrifices himself to save his two sons--he upgrades B-4's programming, and transfers Data's memories into his own body (also granting Data Soong's memories).
Data is back and has a new, more human android body, equipped with proper pink skin and a full range of human emotions. However, he's not sure if he can really be considered the "same" Data or not, since it's a different body but the same memories. Knowing now that his father disapproved of his Starfleet career (since he has Soong's memories), he decides not to accept captain Picard's offer to rejoin Starfleet, and instead wants to track down Emil Vaslovik--so he can bring his daughter Lal back to life.
Review: I'd rate this book 3.5 stars. While the beginning and end were quite intriguing, the whole middle section recounting what Soong was up to during the events of TNG (in first person) really drags on. In fact, a lot of the story is really nothing more than an infodump of what happened during the Soong android TV episodes, with the gaps filled in a bit.
That being said, the story feels totally plausible. It's actually hard to believe Soong never returned as an android in the show itself--everything was set up perfectly, with Ira Graves first showing it was possible to transfer one's mind into an android brain in season 1, and Juliana Trainer being revealed as a perfectly life-like android created by Soong in season 7--it's only logical that Soong would be tricksy enough to fake his own death and copy his mind into an android as well. I was assuming this story would take the easy way out and just "fix" B-4 so he would essentially become the new Data, but having Soong sacrifice his own android body to save him was a much more interesting and poignant solution--and I was elated to see Data back... But how much will this new "Data" resemble the old one?
I really disliked that Choudhury was killed off though. I was just thinking to myself "wow, she's probably the first book-only character that's really starting to make a real footprint" and then DEAD. It seems like such a waste, but I guess every series has to be as "shocking" as Game of Thrones and The Walking Dead these days.
Also, the weird guy from the android-club that kept showing up at random intervals never amounted to anything--though I assume he'll turn up later in the trilogy.
One other small thing I really liked though was how Mack explained that Data ~did~ have emotions back in his Starfleet days--Soong programmed him with basic emotions like sympathy, wonder, etc., that Data just never really thought of as true, deep emotions, but they were there all along.
Overall: I liked it, but didn't love it, just because it dragged on a bit too much. I'm hoping though that, like the "Destiny" trilogy, it will get better with each passing book.