The distressingly plausible premise of “Elysium” is that in the future, after Earth has become a polluted, overcrowded nightmare, the rich have built an idyllic colony for themselves on an orbiting space station, leaving the rest of us to fend for ourselves in our slums while they relax in their artificially perfect biosphere. They have better stuff up there, too, including a tanning-bed-sized medical device that can instantly cure any disease or repair any injury. Back on Earth, healthcare isn’t even as good as it is right now, and people dream of going to Elysium just to be healed. Elysium is an afternoon’s journey from Earth via space shuttle, but only Elysian citizens are allowed to enter. It’s basically a metropolis-sized gated community.

This is the setup for what proves to be a flat and inert sci-fi action story, a disappointing second film by South African writer-director Neill Blomkamp. His debut, “District 9,” started as an apartheid allegory and got less interesting the more it got away from its message; “Elysium” stays on-point the whole time and fizzles anyway.

It stars Matt Damon as Max, a rough, neck-tattooed ex-con who’s gone straight but, like most of his fellow Earth residents, despairs of ever going to Elysium. Places like that aren’t for people like him. He has a low-paying job at a factory that makes some of the defense technology Elysium uses to keep his kind out, under a haughty Elysian boss (William Fichtner) who hates having to work on this filthy planet and views his employees as interchangeable cogs. When Max is exposed to radiation that will kill him in a few days unless he gets to one of those tanning beds, he urgently pursues all available means, including shady ones, of traveling to Elysium. In the process, he reconnects with a childhood friend, Frey (Alice Braga), who gives him another incentive: her young daughter is dying, too, and desperately needs Elysium’s marvelous healthcare system.

A larger plot unfolds pertaining to Elysium’s heartless defense secretary, Delacourt (Jodie Foster, doing some kind of inexplicable accent), who wants to seize control of her government from its president and enact stricter laws regarding immigration. Specifically, she thinks anyone who tries to enter Elysium without authorization should be killed on sight, not apprehended and deported. (You probably have an uncle who feels the same way.)

So it is an allegory about immigration, and about the 1% vs. the 99%. Very timely, you’ll agree. And there are flurries of engaging action. I got a kick out of Sharlto Copley’s maniacal Elysian sleeper agent doing Delacourt’s bidding on Earth, and Blomkamp conceives some nifty futuristic weaponry that belongs in a much more exciting movie. But the stakes feel low. The ultimate goal is for everyone to be granted Elysian citizenship so they’ll all have access to the healing machines — you know, because the Elysians will suddenly become compassionate and cooperative just because the immigration status on people’s passports has changed. The goal ought to be closer to the reverse of that: for the medical technology to be made available to everyone, regardless of citizenship. Naturalization does not make for a thrilling plot device. Also, as I’ve always said, beware of any movie whose climax involves downloading information into a computer before time runs out.

C (1 hr., 49 min.; R, a lot of harsh profanity, moderate violence with some gory images.)