"Finding Dory" is better than I expected it to be because the other Pixar sequels not called "Toy Story" have been quite underwhelming. That said, Pixar is basically using the same formula for "Dory" that it did with "Cars 2" - take an obnoxious supporting character that worked in small doses and build an entire movie around them.
I have no hate for Ellen Degeneres, but Dory grated on me in "Finding Nemo" and it's not much better here. In "Finding Dory," the blue tang fish with short term memory loss who only remembers things when it's convenient to the plot happens to recall that she lost her parents as a baby. So she sets out with her friends Marlin and Nemo to find her folks, and she winds up being taken in by workers at a sort of non-profit sea life rehabilitation center that is very adamantly not Sea World.
As the title would indicate, this is Dory's movie as she tries to piece together where her parents are. Marlin and Nemo set out to find her, but they basically have no bearing on the plot whatsoever. We just occasionally check in with them as they meet new characters and follow Dory's trail.
Speaking of new additions, the MVP of this movie is by far Ed O'Neill from "Modern Family" as Hank the Octopus - a tremendous vocal performance that gives his scenes urgency and heart.
"Finding Dory" definitely has sequel problems with plot points and characters recycled from the first (another story of a fish taken and trying to escape, and getting help from a prickly but lovable old-timer. But the movie is never boring. It moves along at a brisk clip, a lot of the one-liners and site gags work, the animation is beautiful, and it's all very cute without ever having much emotional depth. And that last point is what will keep "Finding Dory" from being among the best Pixar movies like "WALL-E," the "Toy Storys" and "Ratatouille." Despite the ocean setting, it's pretty shallow and Pixar has trained us to expect more.