Thirteen years after Pixar Animation Studios’ “Finding Nemo” made a huge splash, a not-so-splash-worthy sequel docks in theaters this weekend.

The filmmakers overstuff the new undersea adventure, “Finding Dory,” with action and breakneck plot turns. But the sequel isn’t nearly as affecting or amusing as its blockbuster predecessor, a modern classic.

The belated “Finding Nemo” follow-up features several returning characters and voice talents. One of them has been promoted to leading lady. Ellen DeGeneres — TV talk show star, comedian and Metairie native — returns as Dory, the forgetful, freckled blue tang fish.

In 2003’s “Finding Nemo,” Dory is a delightful, funny, major supporting character. DeGeneres gives the character her all again in “Finding Dory,” but Dory’s appeal falls short.

Dory is a one-joke character, afflicted with short-term memory loss. The supporting player scenes Dory enlivens in “Finding Nemo” are fresh and funny, but the humor turns stale during the 100-minute film.

In the new film, Dory again teams with Marlin, Albert Brooks’ stressed-out clownfish. The pair previously journeyed to Sydney Harbor to rescue Marlin’s son, Nemo, from a dentist office fish tank. This time around, they swim from their home at Australia’s Great Barrier Reef to California.

Dory, having suddenly remembered that she has a family, believes her parents are on the California coast. Nemo (Hayden Rolence) comes along, too.

In the first film, Marlin and Dory’s comparatively short trip to Sydney Harbor is an incredible and, for Marlin, character-building journey. But the vast ocean trek that Marlin, Dory and Nemo take in “Finding Dory” is but a blip on the screenplay’s radar.

Flashes of memories from Dory’s past come to her throughout the new story, helping her follow the trail to the loving parents she lost when she was a small fry. Along the way, there’s a cameo from a “Finding Nemo” favorite, Crush the hippie sea turtle. It’s disappointing that there’s so little of Crush in the new film.

Dory eventually realizes that California’s Marine Life Institute rehabilitation center and aquarium is her ultimate destination. Resourceful, persistent and brave, Dory, with help from some new allies, eventually penetrates the facility.

The funny supporting characters in “Finding Nemo” include a trio of Aussie-accented sharks and a pair of British-accented sea lions. Lounging on a rock in the bay, the lions Fluke and Rudder (Idris Elba and Dominic West, respectively) bark up the film’s best comic relief.

Inside the Marine Life Institute, Dory meets Hank the octopus. Hank is the major new character here. Ed O’Neill (“Modern Family”) speaks the part of the cranky chameleon and acrobatic escape artist. The desperate Hank, who’s missing a tentacle, wants nothing more than to not return to the sea and its perils.

Hank and Dory share the movie’s best dialogue. They build a bond as they slink undetected throughout the Marine Life Institute. On the other fin, Hank — in his voice and personality — is so similar to Marlin that he’s redundant. One of them is a clownfish and the other is an octopus, but Dory and Hank’s scenes play like Dory and Marlin, part two.

The confines of the marine institute, filled with sea creatures though it is, are less interesting than the open water that gave “Finding Nemo” its engaging adventures and encounters. By contrast, some richly colorful late scenes in “Finding Dory” show how much oceanic beauty the new movie misses by setting so many scenes inside a building that’s merely by the sea.

A pair of other Dory allies at the institute — Bailey, a beluga whale (Ty Burrell), and Destiny, a nearsighted whale shark (Kaitlin Olson) — play more into the film’s busy, convoluted third-act action sequences than its humor. The visualization Pixar created to show Bailey’s sonar skills looks bad on screen.

“Finding Dory” contains a few moments that approach Pixar greatness and some modest amusements, especially in DeGeneres’ performance and those wacky sea lions. But this sequel is overrun with action. Leaner action sequences and a streamlined screenplay would have brought Dory’s quest in focus and align it closer to Pixar’s many classics.