Every bit as bad as you’d fear of a movie from the writer of “The Book of Eli” and the director of “The Last Airbender,” “After Earth” is incredible—though not in a good way. Where are the guys from Mystery Science Theater when you need them?

The script, developed by Gary Whitta from an idea by star Will Smith, is about at the level of a half-hour Saturday morning kids’ cartoon. Smith plays Cypher Raige, the head military man of the extraterrestrial colony on Nova Prime, where humans took refuge a thousand years earlier after the earth had been made uninhabitable by pollution. His teen boy Jaden plays Cypher’s son Kitai, a youth tormented by memories of the death of his sister at the hands of a beast called the Ursa and by his dad’s long absences. And just as Cypher is returning home for a visit, Kitai is drummed out of Rangers school.

To rebuild bridges with Kitai, Cypher invites him along on his new mission. Apparently their ship is transporting an Ursa to a remote locale, but they never get there, because they’re caught in an asteroid storm and crash on—you guessed it, earth, which has evolved into a place terribly hostile to humans. They’re the only survivors, and Cypher is incapacitated. Their only hope is the ship’s space beacon. But it’s in the tail section, which lies about a hundred kilometers away. If they’re to survive, Kitai will have to prove his mettle by making his way through dangerous territory to the debris, find the beacon and get it to function (which, as it turns out, requires climbing a volcanic mountain). And though Cypher will be able to communicate with him, the heavy lifting will have to be done by the boy.

Every father wants to indulge his children, of course, but “After Earth” is really little more than an expensive vanity project contrived by Will for Jaden. (He goes so far as literally to salute the boy at the close, a proud daddy in both guises.) To put it bluntly, the kid isn’t really up to the challenge, even if his character obviously will be to his. There are endless scenes of him running through the forest and climbing hills—all of which prove that Jaden is in fine physical trim. But even when he’s sharing scenes with others, he shows minimal acting ability, and in the far more numerous ones in which he’s alone—sometimes having to deliver frenzied soliloquies about his conflicted feelings toward his dad—he’s simply embarrassing. Meanwhile Will seems to be phoning in his performance, keeping an imperturbable demeanor even when he’s ordering Kitai to “Take a knee!” in order to calm himself. I guess soldiers in battle find that as useful a technique as footballers on the field.

The stars can’t be entirely blamed for the stiffness of their declamations, though; it would be hard for anybody to deliver the stilted dialogue with conviction, especially when it wanders into modes of expression that are a bit off by modern standards. (Of course one would imagine that the language would have changed a lot more in a world where Cypher and Kitai have apparently become common names.) Neither is helped by the pedestrian direction of M. Night Shyamalan, who was once a name to be reckoned with but has become something close to a laughingstock after “The Village” and “Lady in the Water”—and who helped shape the final version of the script, too. He doesn’t manage to instill much excitement, or even energy, into the film.

And if the human performers make little impact, the CGI critters—baboons, a giant bird and that fearsome Ursa—have even less. They don’t look appreciably better than the late, great Ray Harryhausen’s pioneering stop-motion creatures. They’re part of an effects exhibition that’s far from special, with that storm (and other light displays) distinctly unimpressive and even the futuristic buildings on Nova Prime and the interiors of the ship coming across as cheap, fragile plastic. Apparently a hundred million dollars doesn’t buy what it once did.

In fact, what it’s purchased this time around is a summer debacle that makes you think of what Ed Wood might have come up with if he’d been given a real budget.