The movie is set sixteen years after the events of the first movie and sees the return of the "sky people," aka the Resources Development Administration (RDA), to Pandora. Their mission is to colonize Pandora but to do so, they realize they will have to take out Jake. We learn that Jake and Neytiri have a family. When their family is attacked, they flee the forest and take refuge with a Na'vi clan called the Metkayin, who live next to the ocean and have a symbiotic relationship with the sea-dwelling creatures. We also learn that the Metkayin have a different physiology and a completely different way of life than the forest-dwelling clans. Of course, they cannot stay hidden forever, and another epic battle with the sky people ensues.
There are a couple of different versions of the 4K sets. This one is a three-disc version with a UHD disc and two regular Blu-Ray discs. The UHD disc and the first regular Blu-Ray disc contain the movie, while the second regular Blu-Ray disc has all of the bonus features. The most extensive bonus feature is a series of making-of featurettes that can be played together or individually, totaling almost three hours. The behind-the-scenes material covers the new characters, the returning characters, the casting process, the advances made to the motion capture technology, the stunt work, and more. Then, there is a music video and some promotional spots for the movie. So, this set has well over three hours of bonus content. There is a four-disc set that includes a second bonus disc, and that one has over eight hours of bonus features. It sucks that there are two different 4K releases, but if you want all of the bonus content, you need to pay attention to what 4K set you are purchasing. They did a similar 4K release for the first Avatar movie, but the difference is that the upgraded set for the original movie also had the expanded edition of the movie, whereas, for the sequel, both sets just have the theatrical release (which is on its own over three hours long).
Ultimately, this is a good sequel. It touches on many of the same themes as the original movie and expands the world of Pandora beyond what we saw in the first film. The acting and writing are very good, and the motion capture and CGI technology have advanced so much and look so good that it is almost hard to tell what is practical and what is computer-generated. The A/V quality is top-notch, and the video and sound are reference-quality. If you enjoyed the first movie, this is absolutely worth watching.