You can't explain the plot of Thor: The Dark World without getting trapped in a Tolkien-ish morass of ridiculous-sounding words. See, thousands of years ago, there was a dark elf named Malekith who used an unstoppable zombie warrior named Kurse and an evil, drippy CGI space-substance called Aether to battle Odin's father, Bor, on the day when the nine realms of the world tree Yggdrasil aligned in something called the Convergence. Now, as the Convergence is beginning again, the Aether has infected Jane Foster, the earthly love interest of Odin's son Thor, and Malekith has returned to destroy the nine worlds. See? It basically sounds like a garbage stew of words plucked from Norse mythology, a bad Dungeons & Dragons campaign, and the liner notes of a Led Zeppelin album.
But the good news is that there's a lot of fun here, too. Besides the obvious joys of watching medieval battles between gleaming golden Asgardian soldiers and a bunch of goblins armed with rocket launchers, The Dark World is booby-trapped with funny gags, entertaining fights, and a pair of charming actors who add a human element to the fantasy trappings. Tom Hiddleston as Loki, imprisoned on Asgard after trying to take over the world in The Avengers, and Chris Hemsworth as the arrogant-but-caring Thor have to team up to stop Malekith, and their half-brotherly rivalry is pointed enough to make us forget about the tremendous cast that's otherwise wasted. (Tip to Hollywood: Don't cast Idris Elba and give him nothing to do, and also don't cast Christopher Eccleston as the main bad guy if the dark elf in question doesn't have a personality besides KILL EVERYTHING.)
A whole lot of stuff happens, and a fair amount of it is entertaining, resulting in a Marvel movie that's not as good as the very best of the Iron Man and Avengers films, but that's at least a huge improvement over the first Thor movie.