The plot is simple: A year after the first movie, Dory is living happily with Marlin and Nemo, when her faulty memory suddenly allows her to remember the parents she lost years ago. The three fish cross the ocean and search for her parents in the Marine Life Institute, a fish hospital in California. They meet new characters and run into several mishaps along the way, all of which are very amusing, leading to a glorious, over-the-top climax that still fills me with glee.
But the best thing about this movie, the reason that I'm glad it exists, is the focus on Dory. In the first film, Dory's memory loss is just a running gag. This movie examines the true cost of such a disability, all the problems and fears it brings Dory and the loving parents who struggle to help Dory navigate the world. The devotion of Dory's parents is so heartwarming and life-affirming, and wonderfully highlights the struggles and joys of raising a special needs child. I've seen this movie twice now, and there's a part of this plot line that made me cry both times.
Overall, Finding Dory is not on the same level of Finding Nemo--the characters and situations just aren't as memorable. However, few movies could best Finding Nemo for me, so this is in no way a condemnation--and I will point out that it was better the second time, so this movie could very well become just as iconic with a few years of hindsight. It's a very worthy sequel to the original, with some very funny moments and a wonderful message and a great, crazy ending. Very much recommended.
Bonus: The short before the movie is gorgeous.
Just look at this: