As the Marvel Cinematic Universe expands to cosmic battles and an impending "Civil War," the introduction of its shrinking superhero is surprisingly yet suitably small in scope. While Scott Lang's mission could have worldwide implications, "Ant-Man" is essentially a heist movie, and thereby exactly the kind of genre variation Marvel needs to keep its franchise fresh. But "Ant-Man" would have been better had it kept focus on the heist.

Paul Rudd delivers his goofy affability to Scott, an impulsive thief with a heart of gold. Fresh from prison, Scott needs quick cash to reconnect with his daughter. But busting into some old coot's vault unexpectedly throws Scott into the fray of being a thief for the forces of good, as Ant-Man. Guided by the original tiny titan (Michael Douglas as Hank Pym, Marvel Comics' first Ant-Man) and his daughter Hope (Evangeline Lilly in a distractingly bad cut and color), Scott must break into a shady tech company and destroy its shrinking, weaponized Yellowjacket suit before it falls into the wrong hands.

With his willingness to play the fool, Rudd makes for a fun addition to the MCU's ever-growing lineup of superheroes. Unfortunately his hero's journey is bogged down by the bloated family drama of the Pym clan. Passing his tiny technology and ant-wrangling techniques on to Scott has earned Hank the deep-set chagrin of his humorless Hope, whose secret superpower is sucking all the joy out of a scene. Amid Scott's training montages, father and daughter bicker about the mission, the science and the past again and again, occasionally sounding off on secrets so personal that it's jarring Scott is wedged in as their snarky witness.

While Douglas gets some mileage out of playing the haunted mad scientist, Lilly brings little to the film beyond enviable cheekbones. It's not entirely her fault. Hope's main task is to nag the male heroes, then turn on high heel to prove Scott's romantic reward in a final kiss that feels beyond forced. As much as I crave a greater female representation in the MCU, I could have done without Hope and her sulking altogether. She feels crudely shoved in from some other, more serious, movie. But "Ant-Man" hits its stride in act three.

Without getting into spoilers, the final showdown between Scott and the power-hungry Yellowjacket offers some of the most inventive action scenes Marvel has yet offered. The pair's ability to grow and shrink not just themselves but props around them makes for a high stakes battle that becomes hilarious when played out in its teeny tiny settings like a children's Thomas the Tank Engine playset.

For his part, Corey Stoll makes a decent baddie, adding fits of wild flare to shiny-suited megalomaniac Darren Cross. But as Marvel villains go, he's mediocre at best. Part of the problem is the film's fractured structure. The first act races by, introducing characters so hastily it feels like scenes are missing. Then we hit act two at the Pym home, and everything slows to a crawl of backstory, bad feelings, and mumbo jumbo movie science.

Though Darren Cross/Yellowjacket's motivations are clear, his shoddy introduction never grounds the character. And things just get sloppier as "Ant-Man" grows by clustering in more and more supporting characters. Some parts --l ike that of Bobby Cannavale, David Dastmalchian, and Judy Greer in her third infuriatingly minor role in a summer tentpole following "Tomorrowland" and "Jurassic World" -- are so underwritten they feel like a waste of screen time and talent. Another steals scenes so beautifully you might forget who the real star here is supposed to be.

Playing Scott's ex-con buddy Luis, Michael Peña's is the performance people will be talking about. You want proof that opening up Marvel movies to diversity makes for better movies? Look no further than the casting of Peña as a very atypical thief, who has equal enthusiasm for stealing smoothie machines as he does Belgian waffles, wine tastings and abstract art. Whether he's whistling "It's a Small World," or knocking out bad guys, Peña is so radiant with charm and humor that he totally outshines Rudd. It's so easy to imagine a shorter, sharper "Ant-Man" -- more interested in the heist and this funny pair of bubbly burglars -- that it's a bit maddening.

So where does "Ant-Man" stack up in the collection of Marvel movies? Well, after all the very public battles through production, it turned out better than this critic anticipated. Yet its first and second acts are wildly uneven, vacillating between overeager drama and sprightly comedy. The plot is as peppered with plot holes as the pacing is wonky. When certain jokes fall flat, it's easy to pin it on director Peyton Reed, assuming he's fumbling some Edgar Wright element. But for all these foibles, "Ant-Man" is still pretty damn fun with several excellent action sequences, and stupendously silly gags. I'd actually recommend it for Peña and the extended third-act fight scene alone. In short, (pardon the pun) "Ant-Man" is no "Avengers," but it's an entertaining -- albeit offbeat -- Marvel offering.

"Ant-Man" opens July 17.