In a Nutshell
If you started with Divergent and continued with Insurgent, you probably eagerly awaited the release of Detergent… er… Allegiant. Partly because you want to know what's going to happen to your favorite characters (and a few of the ones you hate, like Marcus), but also because of those blasted cliffhangers that pretty much demand you continue or be left hanging by your fingertips forever. You'd need a heck of a manicure after dangling there for that long.
Veronica Roth finally concluded her hit trilogy with Allegiant in October 2013 (nail file and buffer not included). The book had a massive two-million-copy first press run, and sold almost a quarter of those (455 thousand books, if you're counting) on day one, breaking records for its publisher (source).
In Allegiant, Tris and Tobias finally leave the city and figure out what's outside that stupid fence. Spoiler alert (and by "spoiler," we mean it should be completely obvious): things aren't what they seem, and our heroes get wrapped up in yet another conspiracy that—cue the Moviefone guy—threatens to change their lives FOREVER.
Speaking of movies, Allegiant has already been greenlit for a movie before the film adaptation of Divergent even hits the big screen (source). We guess they're counting on all those 500,000 fans showing up to all three films, despite the series's controversial ending. See, fans are divided about Allegiant because it employs a dual narrator perspective (with Tobias taking over half the book from Tris) and concludes with a crazy-go-nuts ending unlike anything you've ever seen in young-adult fiction.
Don't worry, we won't spoil the ending here. (We will spoil it elsewhere, so look out for our spoiler alerts.) But come on, if you've read through over a thousand pages of this trilogy already, don't you want to figure out how it all ends? Read Allegiant so you won't be the only one not talking about it.
Why Should I Care?
At this point, you either care about the Divergent trilogy (along with millions of other readers and people who saw the movie and have pretended to read the books), or you don't. If you don't… well, Allegiant might make you care.
We mentioned the dual POV above. Allegiant lets readers go somewhere they've wanted to go since the first book: inside Tobias's mind. He narrates a good half of the book, allowing you a first-hand glimpse of what makes the dude sometimes known as "Four" tick.
We also finally get to go past the fence and see what the outside world is like in the post-apocalyptic world of Divergent. Is it a wasteland filled with vicious cannibals? Is it overrun by lovesick zombies ? Populated by kids killing other kids for entertainment? Run by women who think they're characters in a Regency romance?
Honestly, you'll just have to read it and find out.