I'm doing my best to write this without spoilers, but there may be some clues...
First off, the story: Agent J has to go back in time to change something so that Agent K doesn't die, oh and also to save Earth. (vague enough?)
I'm not gonna lie, I like this movie. People were properly in character and there was a good balance between comedy and realistic...ness.
It was a BILLION times better that MIB2, so all of you people worrying about that (like I was) you can freely go see it now.
As for the villain? He's annoying enough that you're on the good side and want him dead, but not so annoying that you want him off the screen.
There was also one part that reminded me of a scene from one of the Back to the Future movies (when you see it you'll know what I'm talking about).
And it came with a very nice ending. VERY nice. Satisfying even!
I would like to say more but then I'll turn into a spoiler machine, so I'll stop at that :D
I give this movie 8/10!