Diving back into the sea is easy in Pixar’s “Finding Dory.” Clownfish Marlin (Albert Brooks) and Nemo (Hayden Rolence), as well as their blue tang friend, Dory (Ellen DeGeneres), are at once familiar and lovable. But this time, their roles are reversed  — Dory headlines this story, and Marlin and Nemo play supporting roles.

After spending much of her life struggling with short-term memory loss, Dory suddenly remembers her long-lost parents and sets out on a journey to find them. Her search leads her to the Marine Life Institute in Monterey, California, where injured sea animals are rescued, rehabilitated then returned to the wild.

After finding her way into the institute, Dory enlists the help of a grouchy octopus, Hank (Ed O’Neill); a near-sighted whale shark, Destiny (Kaitlin Olson); and an echolocation-challenged beluga, Bailey (Ty Burrell). These instantly endearing characters are just as delightful as old favorites, like Bruce and Crush.

“Finding Dory” is not exactly fresh, but it’s never dull either. The aquarium offers a variety of obstacles for the characters, including handsy children and overly diligent employees. Hank’s camouflage proves useful when he and Dory have to hide in plain sight, while Bailey’s echolocation helps Dory navigate the institute.

Some of the fish’s dicey escapes from aquarium tanks push the bounds of believability. Sure, this is a movie in which fish, seals and whales talk and read English, but their getaways are absurd enough to momentarily take you out of the action. For example, Hank miraculously steers a runaway truck down a highway without seeing the road ahead, and the aquarium pipes are oddly accessible, allowing Dory to navigate from a tank to the open ocean with ease. Nonetheless, these scenes are enjoyable thanks to their momentum and inventiveness.

As cheery as it can be, “Finding Dory” has a lot of sadness, for Dory’s memory loss is  portrayed as tremendously debilitating.  Flashbacks to Dory’s youth show her mother (Diane Keaton) and father (Eugene Levy) trying to cope with her disability.

At first, it’s cute to see a baby Dory tell strangers she has “short-term remember-y loss,” but with each time she repeats the phrase, her feelings of inadequacy slowly emerge. Since few fish are willing to empathize with her condition, Dory is — beneath her carefree and bubbly exterior — fundamentally lonely.

Yes, “Finding Dory” is about a family reunion (bring tissues), but it’s also a story of self-discovery. During her action-packed adventure, Dory learns she’s more than a disabled individual.

Bringing the emotion to Dory’s story is DeGeneres, whose voice work hits all the right notes. It’s dripping with unbridled sweetness, but it’s also weighed down by a sense of alienation. DeGeneres’ performance feels emotionally honest, and it’s not hard to imagine her drawing from her own experiences for her reprisal of Dory. For anyone who doubted whether Pixar had another story to tell about the characters of “Finding Nemo,” DeGeneres’ Dory will change their minds.

• “Finding Dory”
• Running Time: 103 minutes
• Rating: PG
• Score: 4/5 stars