We have no idea why it’s called Planet 51, or why the pleasantly blobby green humanoid aliens wear no trousers – jackets and ties or T-shirts for the males, and of course the ladies wear skirts (and have feet shaped like high heels, like Barbie!). We have no idea how cultural convergence allows them to speak English – except for the proper names, which all sound like “Vernkot” and “Glipforg” – or to be having their 1950s, complete with malt shops and alien invasion movies, just as a human from Earth arrives to terrorize them.

It’s not supposed to matter, I suppose. It’s just a kids’ movie; we’re meant to understand. And it doesn’t matter, I guess. Because Planet 51 isn’t really interested in saying much to its audience, whoever that audience turns out to be. As usual with movies like this, much of what makes it as mildly amusing as it is will go right over kids’ heads, like the Singin’ in the Rain reference. And much of what might have made it appealing to true devotees of science fiction and cinema, like how it’s a pastiche of 1950s B-movies, is lost when its parodying of the paranoia and xenophobia of those films is so relentlessly trite and obvious; like the similar Alien Trespass, from earlier this year, we see again that it’s not enough to point and laugh at the silly old movies with their now blatant subtexts – we already got that, thanks very much.

AS USUAL WITH MOVIES LIKE THIS, MUCH OF WHAT MAKES IT AS MILDLY AMUSING AS IT IS WILL GO RIGHT OVER KIDS’ HEADS, LIKE THE SINGIN’ IN THE RAIN REFERENCE.

See, American astronaut Captain Charles T. Baker (the voice of Dwayne Johnson) arrives on this distant planet expecting to find it uninhabited – the best spark of cleverness in Joe Stillman’s (Shrek, Shrek 2, Beavis and Butt-Head Do America) script is how the advance probe we sent is interested only in rocks, and sends pictures home of nothing but rocks, and so neglects to notify us of the planet’s lush and abundant biosphere. Instead, Baker lands his Apollo-style lander in the small-town backyard of Lem (the voice of Justin Long), a young man still living with his parents – and in love with Neera (the voice of Jessica Biel), the green girl next door, natch – who just got a job at the local planetarium.

Now, Lem and his comic-book-nerd, sci-fi-obsessed pal Skiff (the voice of Seann William Scott) must hide the alien invader lest mean ol’ General Grawl (the voice of Gary Oldman) capture the alien invader and let his scientist, Professor Kipple (the voice of John Cleese), remove Chuck’s brain for examination. And maybe win Neera’s heart – or whatever these people use for a circulatory organ that also represents romance – along the way.

You don’t need to have seen the movie to guess how it plays out. You probably didn’t even need to read that explanation; you already knew it would fall out along precisely those lines.

At least the humor that’s tacked onto the story is gentle and unassuming, with only a scant few jokes about the presumed anal violation the alien idiot flyboy Right Stuff astronaut will inflict upon the natives. And there are real delights to be had. Though it’s rarely enough to keep me happy, here I’m not at all uncomfortable calling this “eye candy,” and meaning it in a positive way.

The trio of first-time directors – Jorge Blanco, Javier Abad, and Marcos – along with their teams of designers and animators have whipped up a charming-to-look-at world, an alien Hill Valley that is all round edges and hovering cars, and have invented two of the most delightful only-coulda-been-animated characters ever, in a dog that’s like a cuddly little Giger Alien and in that rock-loving Rover, who also behaves like a dog though it looks like Wall-E. And when these two critters get together, it’s the most adorable puppyish fun you can get without, you know, actual puppies.

I wish the whole movie had been about them.

PLANET 51 (2) • Directed by Jorge Blanco • Voices by Dwayne Johnson, Jessica Beil, Justin Long, John Cleese, Gary Oldman • Rated PG • At Northridge Cinemas, Maya Cinemas.