It's been a year since the events of The Avengers. Bifrost has been rebuilt, and the Aesir are only now done world policing the other realms back to peace. Things won't remain quiet for long - a primeval race, the Dark Elves, return, determined to bring all of existence back to the nothingness before there was light. An oft-used, abstract "villain goal" - here visualized well, and made quite literal; part of their arsenal(the only advanced weapon in this that isn't so bland that it's downright distracting) is a grenade that creates a tiny black hole, relentlessly pulling away those caught within the blast radius. To this end, the leader Malekith(Eccleston bringing out the big guns for the least compelling bad guy of the current cinematic Marvel-verse), with the aid of his lieutenant, Algrim(Akinnuoye-Agbaje, using his immense physical presence to bring a seemingly unstoppable force of destruction to life - a lava-infused bull-like being), seek the Aether, a powerful essence that enables the wielder to drain life and light.
It has recently resurfaced - in fact, it infected Jane Foster(Portman, cute in her awkward social behavior and obsession with discovery; she and our titular hero are pining for one another - with that said, the romance was better in the first one). This makes her a target, and the Asgardians find that their old foe is extremely dangerous. The only way to save the universe(can you say "raised stakes"?) appears to be for Thor(Hemsworth, charming, strong and, yes, arrogant - but now with an appreciation of planning and not rushing in) forming an uneasy alliance with Loki(Hiddleston at his most duplicitous, unpredictable... and vulnerable; in short, his best yet), who now faces consequences for his actions, and the two will come head to head over their on-going conflict - in general, this takes care to follow up on what has happened to these characters(in part by trimming the large cast some, not introducing many new faces, and, yes, less overall development for them) so far, a surprisingly personal story in this adventure of an epic scale.
As this flies by with a pace that never overwhelms in spite of opening the until-now-only-ajar door to the vastness of space, the comic relief jokes and gags come in a steady stream, with the "Terminator 2" style humor - easing the otherwise heavy and bleak subjects, without becoming, well, T3, a parody. There is a ton of action(another of the many improvements on the great Branagh film), and it's much more varied - not merely(which doesn't mean they're gone) deities fighting creatures, we now have mid-air combat, jet fighters, chase sequences, even a major attack on a fortress! Aiming for Star Wars, this shows us more of the galaxy, defining the various areas as unique from one another. The grand orchestral score aids this tremendously, as does location shooting, solid FX work including prosthetics - with practical, suits and the like favored over CG. We go on an adventure - away from that which is familiar to either us or the protagonists. The plot is easy to follow, yet with good twists, and everything is properly set up and pays off.
There is a lot of disturbing content, some mild violence and a little moderate language in this. I recommend this to any fan of fantasy, graphic novels and/or Nordic mythology.