Monsters University acts as a prequel, and we get to see Mike Wazowski and James P. "Sully" Sullivan as they were in college, before becoming Monsters Inc.'s premiere scare team. Turns out the two weren't always best of friends, but a series of unfortunate events leads to the two of them being forced to work together to get into the university's scare program, which is the pool where Monsters Inc. picks it's scarers from. Of course along the way the two of them manage not to set aside differences, but to use each other's strengths to overcome obstacle after obstacle. This is one of the most cliche, and non specific plot summaries you will ever read here, but the fact is that Monsters University is so good, to spoil anything would be doing you an injustice.
As you may have surmised on your own already, I totally loved this movie. Loved it. This time around, I got to see it with my kids, in fact I took them on my daughter's birthday, and it was just a perfect night. Shopping, dinner, and a great movie. Monsters U is funny, uplifting, heartwarming, and simply put one of the best times I've had in a theater in recent memory. See it, whether you have little monsters of you own or not.