The mediocre animated comedy The Secret Lives of Pets is based on an original idea by Chris Meledandri, the head of Illumination Entertainment, the studio responsible for the Despicable Me movies and their popular spinoff Minions. That idea? Toy Story but with house pets, which really tests the elasticity of the word "original." For animal lovers, however, the hook ("Ever wonder what your pets do when you're not home?") is mainlined catnip, given the degree to which we already anthropomorphize our beloved kitties and pups (and hamsters and fish and falcons). The fantasy may relieve a little guilt, too: It's nice to imagine them socializing and having adventures rather than sitting around bored, waiting for their owners to return from work.

The Toy Story cribbing doesn't stop there. The film includes furry analogs to Woody and Buzz in the rivalry-turned-friendship between a beloved old dog and an attention-grabbing new one, and the action moves from a contained living space to the chase-filled openness of the wider world. Viewed generously, The Secret Lives of Pets has recognized a winning formula and followed it appealingly, with brightness, pace and a fusillade of slapstick and verbal gags. Yet the Toy Story comparisons are obvious enough to measure all the ways the new film falls short: the dimmed sophistication of the jokes and cultural references, the thinly distinguished characters, the gloppy sentimentality. "It's not Pixar, but it'll do" may get families in the theater, but it's not an inspiring proposition.

Louis C.K. does make an adorable terrier, though, and his caustic tone establishes the Manhattan setting more efficiently than the opening, which lays the Taylor Swift song "Welcome to New York" over shots of the skyline. He voices Max, the happy resident of an apartment building that flutters with social butterflies whenever humans are not around. (The humans leave a lot of windows ajar.) When his owner Katie (Ellie Kemper) adopts a large, fluffy adorable companion named Duke (Eric Stonestreet), Max resents the divided attention and does his best to sabotage his new roommate. Their relationship changes when Duke tricks Max into slipping away from their dog walker, and it's up to Max's friends — led by Gidget (Jenny Slate), a spirited Pomeranian, and Chloe (Lake Bell), a gluttonous cat — to track them down.

The complications pile up. Max and Duke come across "The Flushed Pets," a cult of sewer-dwelling nasties, led by a vindictive bunny (Kevin Hart), that chase them all the way to Brooklyn. The search party expands to include a hawk named Tiberius (Albert Brooks) and Pops (Dana Carvey), an old dog with wheels for back legs who has the connections to help navigate the sewers and back alleys of the city. Their journey would seem extremely unlikely had small fish not pulled off the aquatic equivalent a few weeks ago in Finding Dory.

At a certain point, The Secret Life of Pets drops the "secret" part that was so essential to its appeal. The pets stop acting like pets when human eyes are on them and suddenly a couple of dozen of them are piled into a taxicab blazing across the Brooklyn Bridge. It may be nitpicky to question the internal logic of a film where one dog has ninja skills and another can grasp and turn a set of keys underwater, but the further it ventures into the improbable, the worse it gets. Perhaps because it reneges on its initial promise: These are not how pets act when their owners are not around. This is how they act when they're in a Madagascar sequel.

The best scenes are the precious few domestic ones that bookend the film, when the pets are introduced via a clever series of comic vignettes and the apartment building turns into a bustling social hot spot. They may chat it up, but the pets are still recognizable as animals, in all their particular enthusiasms and desires, and defined sweetly in relationship to the owners who love them. The Secret Life of Pets drifts away from the fantasy that sparked it, until the pets don't seem like pets anymore.

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