Sigma. Is. Back. With Kingdom Of Bones, it looked like Rollins was delving too far into the fantastical and leaving behind the more grounded roots of this series. Here... the ties are more to the scifi than the fantastical, including The Abyss, Pacific Rim, Earthcore by Scott Sigler, and even... Mass Effect 3??? Yes, there is one particular scene roughly 2/3 into this tale that while not *quite* word for word with a particular moment in Mass Effect 3, is damn close - and the sentiments and reasons are identical within their worlds. (To be fair, in this particular situation... the wording is always going to be very similar, no matter where you encounter it.)
But more than the scifi zeitgeist connections here, this tale truly gets back to the real roots that make Sigma Force so special. We've got the historic and the scientific, and again, the scientific is at least more closely based on actual science this time around. But we've also got the camaraderie among the team, including having most of the team (minus Painter, Lisa, and newer team member Jason) together the first time we see them and having a bit of a mini-adventure then as the overall tale begins to pick up. Then we've got the Sigma Split, with the team breaking up to go their own separate projects to try to uncover and stop whatever is happening. Each of their specialties get highlighted and tested to degrees not seen in recent Sigma books in a fair amount of time, even Gray's "special brain". More akin to David Wood's Dane Maddock Adventures in this particular point, there are even several callouts to other characters from prior Sigma tales and how those characters are still impacting the world even through the events of this tale.
And that epilogue... It sets up the 2024 entry into this series to be one of the most explosive in quite some time, and you're going to want *that* book in your hands the moment you finish this one.
Very much recommended.