As I noted in my review of the first movie, this movie is hilarious by itself but better in the movie series.
The first movie helps to set up the characters, brings us a lot of humor, and can truly be watched by itself without seeing any of the sequels.
But, to truly get the most out of the three movies (I don't count the direct to DVD movies that came out later), it's best to see each of them in order.
The first movie has our characters as young and still fairly stupid as far as life is concerned. They learn a bit at the end, grow a bit, and we all get to go along for the funny ride.
This movie has at least one of the group (no spoilers, just see it) already maturing from his role in the first movie and the rest of the characters either resisting the call to adulthood or working their way through their own twisty hilarious paths to maturity.
Again, if you are going to see them, I strongly recommend watching them in the order that they came out in so that you can not only enjoy the humor in each, but so that you can notice that the characters are growing into adults throughout the series, ending in the American Wedding movie.
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