There’s something in the water at Pixar. The computer animation studio is rarely content merely creating cute anthropomorphised wildlife or dazzling frenetic set-pieces, examples of which abound in their latest, Finding Dory. There also continues the sadistic streak that reached new heights during the first reel of 2009’s Up, continued with the coda in 2010’s Toy Story 3 and went beyond the pale tossing the adorable pink elephant Bing Bong into a memory hole last year’s Inside Out. Pixar doesn’t just pluck the heartstrings, they throttle them until they vibrate into higher dimensional space.

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Finding Dory, a same-but-different spin on 2003’s tremendously popular Finding Nemo, is relentless in its pursuit of getting its audience to sob. (Think of it as an interactive film: the characters are immersed in salty water, so the audience should be, too.) These are tremendously effective moments, thanks to the highly advanced machinery that can create nuanced emotional tics on the faces of kooky, googly-eyed creatures and the outstanding voice acting talent of people like Ellen DeGeneres, Albert Brooks and (believe it or not) sitcom dingus Ed O’Neill. This isn’t to say the film isn’t repetitive or facile, it most surely is, but when it moves with its natural currents it is quite a remarkable achievement.

If you never saw the original, or suffer from some sort of memory loss, no need to worry. The silly aide-de-camp from the first adventure, Dory (a Pacific regal blue tang voiced by DeGeneres), is living in comfort with her adopted family, Marlin (a nervous clownfish voiced by Brooks) and his son Nemo (Hayden Rolence), among other happy members of the sea. But we get a look into her childhood, one in which her loving parents (voiced by Eugene Levy and Diane Keaton) do their best to care for her despite a disability. Dory, you may recall, suffers from chronic short-term memory loss. Think of Memento but with gills instead of tattoos. Other than her name and the fact that she is separated from her parents, she can’t remember what it was that she just forgot a moment ago.

That begins to change, though, when she gets bonked on the head and starts seeing flashes. Soon, she and her two clownfish pals are on the trail of mom and pop, which leads them to a Sea World-like institution in California. Finding Dory downshifts into a race-the-clock rescue scenario, and while this may not be too original (see Toy Story 2), it affords the animators room to stretch out and have a bit of fun.

Don’t worry too much about the location: this isn’t a Blackfish scenario. As celebrity spokesperson Sigourney Weaver explains, this is a rehabilitative habitat, in which oceanic life is nursed back to health and returned to the open seas. What that means is: get ready to joke around with sick fish! (Fans of the Albert Brooks Cinematic Universe may want to do cartwheels. You may recall that the 1999 sendup of Hollywood, The Muse, climaxed with Brooks pitching a Jim Carrey vehicle about sick fish.)

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The rules of Finding Dory are a little vague (if you are really sick, you end up sent to a different infirmary in Cleveland?) but it’s more than enough for its primary function: wacky, visually dynamic action scenes involving sea lions, birds, a network of pipes, gift stores, whirlpools and touch tanks under assault from terrifyingly grabby children.

The high point of all this is a character that can stand alongside Cars’ Mater, Sulley from Monsters, Inc. and Wall-E’s Wall-E: Hank the octopus. Grouchily voiced by Ed O’Neill, Hank, who has chameleon-like powers in addition to his Reed Richards-style elasticity, is far on the cartoonish edge for typical Pixar. His exaggerated expressions and movements are absolutely mesmerising. He toggles between goofy and sly, and I tell you his likeness is going to be in our pop culture for a very long time. (Studio PR is already pushing Hank as the most complex animated character created.)

Facebook Twitter Pinterest Mesmerising … Hank and Dory. Photograph: Pixar/AP

But for each moment of ecstasy spent squishing around with Hank, there’s another of panic with the emotionally wounded Dory. As a tiny fish-ling with an adorable speech impediment, her behavioural disability led her to get separated from her family, and she has never stopped blaming herself for letting them down. This is some heavy stuff and, amid all the gags about near-sighted whales, arguably one of the more frank depictions of raising a special-needs child in a mainstream film. DeGeneres’s vocal performance is extraordinary, effortlessly shifting from jokes to self-perpetuated anxiety attacks. One particularly devastating scene, shot in a disorienting first-person manner, might actually be too overwhelming for small children. It was certainly overwhelming for me.

The problem with Finding Dory is it doesn’t know when enough is enough. Its believe-in-yourself message is pounded with the subtlety of a hammerhead shark and the final action sequence is really too far-fetched to fathom. (I know, I know, this is a movie about a fish that can read and suffers from memory loss, but trust me: the ending gets extra stupid.) I recognise there’s thematic consonance here if the movie itself is constantly repeating itself, but let’s not give Pixar postmodern cred where it may not have asked for any. This is a movie with a great number of bright spots, but it’s still for an audience with a short attention span.