Thor: The Dark World is a sequel to both the original Thor movie and The Avengers. It is set a couple of years after the events of the original Thor movie and sees Thor going through the nine realms cleaning up Loki's mess, which is also taking a lot longer because of the Bifrost having been destroyed in the first movie. Thor keeps track of Jane (mostly via Heimdall) but has not seen her in all that time, which makes for an awkward reunion, to say the least. Loki is imprisoned on Asgard, visited only by Frigga.
This movie sees the threat of Dark Elves, with the main antagonist being Makliketh (played by Christopher Eccleston), the leader of the Dark Elves who was defeated by Thor's grandfather. He is trying to capture a substance called The Ether, which can basically undo the Big Bang and bring the universe back into darkness. I will not spoil the movie for the handful of people reading this by now who have not seen the movie, but the plot requires Thor to return to Earth after Jane is put in danger and team up with Loki to take down the Dark Elves. It also sets up the concept of the Infinity Stones, which, of course, play a larger role in the following movies.
For those who get the 4k Disc set, the movie looks and sounds great in UHD. Not a huge jump from the blu-ray version of the movie, but noticeable enough that if you have a 4k TV and blu-ray player, it is worth the double-dip. The UHD disc just has the movie itself, and then the extras are included on the second disc which is a regular blu-ray. The extras include a commentary track on the movie by the director Alan Taylor, Marvel's head honcho, Kevin Feige, and Tom Hiddleston, who, of course, plays Loki. The commentary track provides some good insight into the process of making the film with some good behind-the-scenes details from Hiddleston. Then there is another Marvel's One Shot titled All Hail the King, which is a documentary about Trevor Slattery, the "bad guy" from Iron Man 3, showing what is life is like in prison and setting up the 10-rings storyline. Then there are deleted and extended scenes, a handful of making-of featurettes that can be viewed one at a time or all at once, and then a gag reel. So, a lot of material if you like to look at the bonus material.
Most of the supporting cast from the original movie, including Jamie Alexander, Renee Russo, Anthony Hopkins, Idris Elba, Stellan Skarsgård, Ray Stevenson, and Kat Dennings, appear. Zachary Levi replaced Joshua Dallas as Fandral, one of the Warrior's Three in Asgard, because of a scheduling conflict. The other notable addition to the cast is Jonathan Howard, who plays Ian, Darcy's intern, who is basically a lacky she "hires" to do the stuff she does not want to do as Jane's intern. The movie gets ragged on, but it is definitely not as bad as some make it out to be. It is definitely a filler story, but it does set up a lot of what is to come down the line, so it is essential to what is going to play out in the later movies. The acting is great from the entire cast. I would say that the supporting characters all kind of took a back seat in this movie and were not as involved as they were in the first one, but all of them made the most of their screen time. Kat Dennings stole pretty much every scene she was in, as did Stellan Skarsgård, who had to play Erik as stark-raving mad (with lucid intervals) because of Loki taking over his mind in The Avengers. There are some cameos, including the usual Stan Lee cameo, but also a pretty hilarious one by Chris Evans. The movie continues the mid-credits and post-credits scene tradition. The mid-credits scene set up the Guardians of the Galaxy, and the post-credits scene was a more humorous wrap-up of the movie. While it is not the best MCU movie, it is not as bad as some make it out to be, and is definitely worth watching.