Watched this for completeness. Because (please don't take away my nerd credentials) I've never been much of a Thor fan. It's a matter of taste, the same way I like watching baseball and football, but find basketball and (especially) soccer uninteresting.
It makes sense to me: obviously guys like Captain America, Iron Man, Spider-Man, etc. are more believable and relatable than a Norse god (but not really a god) with family problems, coming from an invisible alternate reality that gets joined up with ours every so often.
Here, the deal is that Thor's girlfriend, Jane, gets occupied by the "Aether", a bit of mumbo-jumbo desperately sought by the evil elf Malekith and his minions, in order to bring darkness upon Asgard, Earth, and all the other realms in the universe. Thor must enlist the imprisoned Loki to aid in his quest to disinfect Jane and defeat Malekith, and… oh, there's a lot of fighting and fantastic special effects, and etc.
But the actors deserve some sort of award for simply making believe (but not making me believe) that this all makes perfect sense. There are many hints that the screenwriters, at least, aren't taking the scenario very seriously. ("I better get my pants.")
It's becoming a Marvel trademark: yet another movie that involves an epic battle in, above, and around a major metropolis. (London, this time.)