“I’ll be home, sitting with my friends, and somebody will say, ‘Heeeey, let’s get small.’ You know, we get small, and the only bad thing is if some TALL people come over.” – Steve Martin, “Let’s Get Small,” circa 1977.

When humans are shrunk in the movies, there’s usually the expectation or at least the hope of returning to full size at the end of the adventure.

In “Fantastic Voyage” (1966), a submarine crew is shrunk all the way down to microscopic size so they can be injected into a prominent scientist’s bloodstream and attempt to repair the damage to his brain.

In “Honey, I Shrunk the Kids” (1989), a scientist accidentally shrinks three kids to the size of insects.

And in “Sack Lunch” (1996), which opened the same weekend as “The English Patient,” an entire family fits into a paper bag and …

Oh wait. “Sack Lunch” wasn’t a real movie; it was a “Seinfeld” universe movie.

Now comes “Downsizing,” and this time around, the process is voluntary and it is irreversible and it is viewed as the last best hope to save the planet.

No for real.

Here’s the intriguing and absurd premise cooked up by director Alexander Payne (“Sideways,” “Election”) and his longtime writing partner Jim Taylor. With the planet only 200 years or so from certain extinction due to the myriad problems caused by overpopulation, a team of Norwegian scientists have perfected a process that can shrink humans to a height of just five inches tall. It just might save us all!

Think about it. If the world’s population is gradually downsized and relocated to domed, climate-controlled communities, we’d be leaving a much, much, MUCH smaller footprint in every way imaginable. (The communities would have to be domed because all the suddenly giant-sized birds and insects flying about the planet would be swooping down, picking off the tiny people and chewing them as snacks left and right.)

When we join the story of “Downsizing,” approximately 3 percent of the world’s population has gone small, and it appears as if those who have taken the plunge are loving it. (For one thing, if you have just an average income in the Big World, you’re suddenly rich in the Small World. You can build a mansion for the cost of constructing a doll’s house back in your old life.)

One such couple about to make the transition: the Safraneks, Paul (Matt Damon) and Audrey (Kristen Wiig), a typical American couple stuck in an early middle-aged rut, with no genuine hope of realizing their dreams in their current life. If they go Small, they can move to a dome-covered community and enjoy an idyllic, luxurious, carefree life.

Through the use of some terrific special effects and humorous explanation, we’re taken through the process of miniaturization. We learn all hair must be removed because hair does not shrink and all fillings must be taken out, because fillings won’t shrink either, so your head would literally explode. (Tragically, the technicians know this to be true because on rare occasions they’ve missed a filling, and KABOOM!)

When Paul wakes up Small, he learns there’s a bit of a complication. At the last minute, his wife freaked out and backed out, and she’s not reconsidering. Good luck, Paul!

After Paul adjusts to his new life as a very small and very lonely man, “Downsizing” remains rooted in the world of the Small for a long stretch and turns into a hybrid of raucous comedy and social commentary satire. Paul becomes party pals with his filthy rich, hedonistic neighbor, Dusan (Christoph Waltz), and he strikes up a friendship that could turn into something more with Ngoc Lan (Hong Chau), a Vietnamese dissident who was shrunk against her will. (Turns out there’s oppression and a class system and poverty and racial issues in the Small World after all.)

For a while, “Downsizing” seems to lose its way, especially in an extended sequence in Norway, where Paul and Ngoc Lan and Dusan meet the scientist who invented the shrinking technique and the original community of shrunken people (who have been Small for more than a decade now). They’re a cult-like group, convinced the end is upon us, and they’ve built an elaborate and relatively massive underground community designed to be the home base for human life for at least the next 200 years.

Paul must decide if he’ll save himself by literally going underground, or if he’ll take his chances and join Ngoc Lan, who has dedicated her Small Life to helping out the poor and the sick and the elderly. By that point, “Downsizing” isn’t really trying to be funny any more, and it feels as if we’re being lectured a bit too much in the home stretch. (Also, it feels like a missed opportunity to have such little interaction between the worlds of the regular-sized and the small in the second half of the movie.)

Damon is in prime everyman mode as Paul, a good guy with a good heart who wouldn’t mind catching a break, a big break, just once. Waltz has a blast playing the party king Dusan, who has some wise observations about the ways of this new world. And Hong Chau is brilliant as the fiery and funny and fantastically blunt Ngoc Lan.

Even at 5 inches tall, she is not to be messed with.

★★★

Paramount Pictures presents a film directed by Alexander Payne and written by Payne and Jim Taylor. Rated R (for language including sexual references, some graphic nudity and drug use). Running time: 135 minutes. Opens Friday at local theaters.