Here's the problem with an "Avatar" sequel: you can't visit Pandora for the first time again. Nothing is as exciting as your first visit to a new place. What made the first "Avatar" such an incredible success was the world-building, not the entertaining but derivative action story. It's James Cameron. You know he'll add a good (but not great) adventure story on top of the world he's already built out. Which means ... you've kind of seen it before. The thrill of seeing Pandora is considerably lessened because it's not new, although he does take us to the ocean and introduce us to many new creatures. But he also recycles some story elements I kind of feel like he shouldn't have: it's science fiction, so Cameron feels entitled to bring back both Quaritch (Stephen Lang) as the big bad, and (via a different method, but also blue) Sigourney Weaver.
The story opens on the Sully family - Jake (now big and blue after the last movie, still Sam Worthington), his wife Neytiri (Zoe Saldaña), their two sons Neteyam (Jamie Flatters), Lo'ak (Britain Dalton), their young daughter Tuk (Trinity Bliss), and their adopted daughter Kiri (Weaver, blue, and twelve years old ... they explain in the movie). And then we have the arrival of a large blue version of Miles Quaritch and several of his henchmen. To save their family, Jake and Neytiri take their kids and run away to an island in the ocean. Where they have to learn a new culture and meet many new creatures, some of which are dangerous. Inevitably Quaritch finds them, the other humans do evil things too, and there's plenty of exciting fighting.
It's just not the revelation that the first movie was.
What follows is kind of a SPOILER - you've been warned. For unknown reasons, Grace Augustine's avatar is still alive in a tube despite her death - and it gave birth to a daughter. That's how we have the ~12 year old version of Weaver. There's some discussion of who the father might be - but I think Cameron's having fun implying that Kiri is effectively a virgin birth, the daughter of the goddess Ehwa? Which would be more than a little heavy-handed, but we have to wait to see how that plays out ...