WARNING: It is virtually impossible to talk about this movie without giving away some character information that some may find spoiler-ish. I don't happen to think it is, and I'm not going to say anything more about the actual story than I ever do, but for those who are obsessively sensitive about such things, there's your warning.
The smartest thing J.J. Abrams did in the first installment of his Star Trek Babies reboot series was to establish that we were no longer in the timeline of the original; history changed, and we've moved into a parallel timeline. That gave him the freedom to tell new stories and introduce new characters without being tied to the oppressive "but that contradicts something that Chekov said in episode 12 of season 3" nonsense that Trek fans can be prone to. So what does Abrams do for Trek Babies 2? He decides to give us his riff on the most memorable villain (and movie) in Trek history.
Yup, Khan is back (played by Benedict Cumberbatch), and the movie opens by essentially putting him in the role of Osama bin Laden. There's a terrorist attack, and we want to capture and punish Khan, who is hiding out on the Klingon homeworld (standing in for Pakistan). We're not at war with the Klingons, but relations aren't good -- some say war is inevitable -- and we certainly don't trust them enough to ask for their help in capturing Khan, so Kirk and crew are sent on a Top Secret mission to get the guy.
And once Kirk and Khan meet, the movie turns into a series of riffs on scenes and bits of dialogue from The Wrath of Khan, given spins and twists and reversals that will no doubt have the most devoted Trek fans deliriously happy and pleased with their own cleverness at recognizing all of them, but don't seem likely to be terribly interesting to series newcomers. We're not really being asked to respond to what's happening on screen, but to our memories of what's happened in earlier versions of Trek. It's a "newbies need not apply" movie; fortunately, the box office suggests that there are enough fans that Abrams can keep rehashing old stories and foes in future movies. (Next up: Kirk Meets the Borg!)
The performances from the central crew are both good and bad, and precisely in the same ways as they were good and bad in Abrams' first movie. Chris Pine and Zachary Quinto are delightful as Kirk and Spock, and their emotional bond is so obvious and so deep that Spock's romance with Uhura (Zoe Saldana) can't help but seem puny and uninvolving by comparison. Anton Yelchin's Chekov is still not much more than a bad accent; Simon Pegg is still playing Scotty as a clownish buffoon. Karl Urban's McCoy is significantly better than in the first movie; he's playing the character's abrasive exasperation at a more realistic level. And John Cho as Sulu is given a bit more to do this time, and handles his big scenes very nicely.
The casting of Benedict Cumberbatch as Khan is baffling. He's a fine actor, and though he isn't given much of interest to do in the movie, he does it well. But these are supposed to be the characters from the original series; certainly the members of the Enterprise crew have been cast to resemble the TV actors as much as possible, both physically and temperamentally. But Cumberbatch stepping into a role orginally played by Ricardo Montalban? Makes no sense, physically or vocally (the accent alone sinks it) and the break between this timeline and the original is recent enough that this Khan really should be just a younger version of Montalban's.
So mixed feelings here, I suppose. If you know the series well enough to catch all of the in-jokes and references, you may well love this; if you're relatively new to the Star Trek world, it may be mildly entertaining, but you're going to sit through it with the nagging feeling that you're missing something. Most frustrating of all is what this movie tells us about Abrams' (and the studio's) vision for the new series. What could have been an opportunity to boldly go where these characters have not gone before appears likely instead to be nothing more than an ongoing greatest hits medley.