It’s ironic that a movie about a character with short-term memory loss turns out to be one of Pixar’s more forgettable efforts. “Finding Dory,” a part-prequel, mostly sequel to the studio’s 2003 smash “Finding Nemo,” turns out to suffer from the same malady that many live-action pictures do when they elevate a supporting character to star status. What works in smaller doses can prove less satisfying at center stage, and that’s the case here.

The character of Dora, the absent-minded fish voiced thirteen years ago (and now) by Ellen DeGeneres, is revealed, in periodic flashbacks to her childhood with concerned parents Charlie (Eugene Levy) and Jenny (Diane Keaton), to be suffering from a physical disability rather than being merely scatterbrained. Despite warnings about not wandering, young Dora is swept away by the fearful “undertow” and winds up alone. Cut to a year later, when Dora has helped Marlin (Albert Brooks) locate his lost son Nemo (now voiced by Hayden Rolence). She begins having flashes of recollection about her childhood and sets out to find her parents, with Nemo and the reluctant Marlin tagging along.

The search quickly takes them, courtesy of sea turtle Crush (Andrew Stanton), across the Pacific from their Australian home waters to the Marine Life Institute on the California coast, where Dory intuits that she last saw her parents. With some advice from lazy sea lions Fluke and Rudder (Idris Elba and Dominic West), who lounge on a rock overlooking the place, Dory makes her way into the facility, where she’s befriended—after a fashion—by Hank (Ed O’Neill), an octopus with only seven tentacles that offers her help in exchange for a tag that will take him to a permanent home at an aquarium in Cleveland that caters to injured aquatic life. Meanwhile Marlin and Nemo sneak into the place, too, and search for her. Also on hand to lend the trio assistance are Destiny (Kaitlin Olson), a whale shark with optical problems, and a beluga whale called Bailey (Ty Burrell), whose power of echolocation comes in handy during the long, action-packed escape sequence that closes the picture and leads, of course, to Dory’s reunion with Charlie and Jenny.

One notes immediately, of course, that this scenario doesn’t literally involve anyone looking for Dory—except, of course, her off-screen parents, who are eventually shown awaiting her return—as much as Dory’s own search for her own identity. That’s certainly a touching idea, but in the event the execution isn’t all that engaging. Technically, of course, the picture is the usual Pixar marvel, with the underwater sequences masterfully rendered, especially in the IMAX 3D format. Perhaps the most eye-catching single effect, however, has to do with Hank’s habit of blending into whatever environment in which he finds himself as a method of camouflage. His color changes provide the movie’s wittiest moments.

Otherwise, though, the script relies too heavily on Hank’s curmudgeonly persona to carry things. One can understand why: Dora, the spacey chatterbox, can get a mite tiresome with her constant “Hellos?” and Marlin and Nemo, despite their orange hue, are a pretty colorless pair this time around. But Hank proves fairly one-note as well—there’s a great deal of Oscar the Grouch to him, and even when delivered by a fellow as adept as O’Neill, his grumbling can get a little tedious. Fluke and Rudder add a few moments of amusement, as does a wordless sea gull that carries Marlin and Nemo about in a pail, while Sigourney Weaver contributes a vocal joke that’s overused but still funny. Unfortunately, Destiny and Bailey on the one hand, and Charlie and Jenny on the other are just nice couples that play more to sentiment than humor.

“Finding Dory” will afford modest pleasure to children and adults alike, but among Pixar sequels it’s more “Cars 2” or “Monster University” than “Toy Story 2,” and it certainly doesn’t approach the level of the studio’s best, like “Ratatouille” or “Up.” As usual, it’s preceded in theatres by a short: “Piper” is about a hungry little sandpiper that overcomes its fear of tidal waves to become one of its flock’s most prolific hunters. Directed by Alan Barillaro, it’s rather like the feature that follows it: beautifully made, but more ephemerally cute than really memorable.