The main thing that you need to know about Men in Black 3 is that it is every bit as funny as the original Men in Black (1997) and much better than the sequel Men in Black II (2002).
If you are not familiar with the series but think you might enjoy a whimsical off-the-wall sci-fi comedy, there’s no harm in starting with this one.
Agent J (Will Smith) and his laconic weather-beaten partner Agent K (Tommy Lee Jones) work for a super-secret organization called MIB. Their job is to suppress evidence of UFOs and make sure that aliens visiting Earth behave themselves.
Boris The Animal (Jemaine Clement) is a very bad alien whom Agent K arrested long ago. He is being held in a super-super-maximum security prison on the Moon, but at the start of the movie he breaks out and goes after Agent K. More specifically he gets a time-travel device, goes back to 1969 and kills Agent K before he can arrest him.
This leads Agent J to travel back to 1969 (a time when alien influence on our culture was at its height) in order to thwart him. There he meets the young Agent K (Josh Brolin) who seems much more voluble. And that’s pretty much it: a setup for elaborate comedic riffs on aliens, time travel and pop culture. And there’s an excellent ending which I found highly satisfying.
I liked the ending very much too. I don’t think it was part of the original story-plan, because there were many opportunities where they could have dropped in little foreshadowing clues and didn’t — but it didn’t feel “tacked on” either. It was a great little twist.