In the alternate dimension of “Tomorrowland,” people swoop through the sky with jetpacks and parents push babies in hovering strollers. Swimming pools are uncontained blocks of water suspended in the air — a whole group of them, one above another, so that swimmers can dive into one then pop out the bottom and somersault into the next.

What’s the purpose of such an invention? Who knows? But it looks dazzling. It’s almost enough to distract from the shapeless story surrounding all the very special effects.

“Tomorrowland” is based on a Disney theme park area, and despite that inauspicious source material, the family-friendly movie has seemed poised to be one of the most exciting entries in the blockbuster season. For starters, it isn’t a sequel. Trailers haven’t revealed much besides brilliant images and vague but rousing narration from George Clooney. But the director at the helm, Brad Bird, brought us “The Incredibles” and “Mission: Impossible — Ghost Protocol,” and “Lost” writer Damon Lindelof helped with the script.

So it’s regrettable to report that the action adventure is a letdown.

The story follows two modern-day scientific geniuses on a quest to get to the vibrant metropolis of Tomorrowland, a secret world filled with big thinkers and grand ideas. But before they can go, they need to tell the audience their back stories, in great detail.

Clooney plays Frank Walker, who, we see in flashback, was introduced to that alternate dimension as a kid at the 1964 World’s Fair. There, he met a girl named Athena (Raffey Cassidy), who gave him a pin that transported him to Tomorrowland. The other genius is a high schooler named Casey (Britt Robertson) — essentially Pollyanna on a motorcycle. In all of her classes, from English to history to science, she hears nothing but doomsday reports. The ice caps are melting, war is breaking out everywhere and our world is basically a dystopian novel. So she raises her hand to ask: How can we can we fix it all? Anything is possible, she reminds us again and again. And again. And again.

Athena, still a little girl after all these years, turns up to bestow a pin on Casey, which also whisks her away, but only for a few minutes — just enough to give her a taste for more. And that’s why she tracks down Frank. She wants him to take her there for a longer stay.

Clooney is both amusing and touching in this role. He calls Casey “kid,” speaking the parlance of movie curmudgeons everywhere, and he resists her optimism for as long as he can. He’s just another apocalypse-obsessed conspiracy theorist — albeit an uncharacteristically foxy one — with a dozen locks on his front door. But Casey reminds him of the childlike earnestness he used to have, and nostalgia is a powerful thing.

Cassidy is also a commanding presence. With her clear, penetrating eyes, she occupies the precocious kid role without being overbearingly twee: Sure, she says cute things, but she’s also a tiny assassin. The characters are all quite vivid, even if they sometimes speak in motivational poster quotes.

So much of the movie consists of the back story that revealing much more feels wrong. Just know that the action ramps up once we reach the present day. Bad guys materialize (though it’s never entirely clear why), including a hilarious robot played by Matthew MacCaull, whose bright, mechanical smile makes him look like a life-size Ken doll.

The camerawork and editing are impressive, especially in scenes when Casey toggles between Earth and Tomorrowland with the touch of her magical pin. Once she arrives in the futuristic city, the camera swoops and circles around her, adding to the sense of wonder. And a scene much like a high-tech version of the “Home Alone” finale is also thrilling to watch despite taking place entirely within Frank’s cramped hideout.

The most problematic part of the movie is the way it keeps the story hidden from the audience. Why do we need to get to Tomorrowland? It’s a mystery! And we don’t fully understand the great importance of the journey until the last chapter of the movie, at which point a minor character suddenly becomes a villain. The narrative is messy and meandering, putting more emphasis on emotional payoffs than driving the plot forward. (It also leads to a somewhat problematic minor thread, as Frank acts like a man scorned whenever Athena is around.)

Maybe the ultimate goal of “Tomorrowland” remains obscure because once you know where the story is headed, you realize it’s a familiar tale. The movie can conjure up futuristic images, but the story is nothing we haven’t seen before.

PG. At area theaters. Contains sequences of sci-fi action violence and language. 130 minutes.

PG. At area theaters. Contains sequences of sci-fi action violence and language. 130 minutes.