This may be a bit of a spoiler, but the first thing Jane Foster (Natalie Portman) does when she sees Thor, is to slap him in the face. Then for good measure, she slaps the other cheek.
Apparently, before the universe was born, there were elves. And not cuddly elves like Legolas in LOTR, but mean looking elves with hideous scars. And they wanted to destroy the universe. But, the Asgaardians were on the case and a mob of bearded men in armor with swords (led by Odin's daddy) were somehow able to defeat the space ships and phaser rifles of the elves and take away their ultimate weapon, the aether (a swirling red blob of evil, but not the red blob of stuff from the Star Trek reboot). They took it away and hide it and forgot where they put it.
Later, much later, Jane Foster is wondering around in London with her assistant, holding what looks like a giant smart phone with the world's worst map app and falls through an invisible hole into somewhere, and finds the aether. And it crawls into her eyes or something. Then everything gets confused. Eventually, Thor shows up, and she slaps him.
What follows is lots of hard to follow dialog between weird creatures, monsters with glowing red eyes, Thor's hammer flying around, treachery, redemption, trickery (Loki, of course), seeming rapprochement (Thor and Loki), death (real and not real), explosions, space ships, half of London destroyed, and kisses (Darcy and the intern "My name is Ian"). The usual.
Anybody who goes to a movie like this and expects anything more broad that good triumphing over evil and anything more subtle and the crashing of a giant hammer, deserves to be disappointed. If you're looking for entertaining spectacle, you'll like it.
You already know that you have to stay through the credits to get the Easter egg at the end. Be forewarned that this movie has 2. After watching a list of credits like this I always wonder, how many people did it take to make a movie like this? I estimate at least a thousand names in the closing credits.
One final thought. Among the many things in this movie that didn't make sense was what the evil elf guy was going to get out of destroying the universe. I guess he had another universe tucked away to escape too, or was he just in a suicidal mood and decided to take everybody and everything with him. He sure was a hateful, pointed ear guy.