Truth be told, “After Earth” wouldn’t exist had Will Smith not cooked it up as yet another star vehicle for his son, Jaden, now 14. But since buying your kid a movie credit is a tradition that dates back to the beginnings of Hollywood, you can’t hold that against it.

This sci-fi adventure about a boy who must become a man to save himself and his wounded warrior father on a hostile world is a corny, generally humorless M. Night Shyamalan picture without his trademark surprises and twists.

It’s a straightforward quest in which incapacitated Dad (Will) sends guilt-ridden, fearful teen son Kitai (Jaden) off to fetch a rescue beacon that broke free of the spaceship they just crashed in.

This is Earth, a thousand years after we’ve abandoned it. The buffalo and trees and vast migrating flocks of birds are back. There are digital beasts in the primeval forest Kitai must cross. A monster named Ursa, which was being transported in the spaceship, is on the loose, too, tracking Kitai’s “fear.” There are rivers to cross and high cliffs he must fly from. The oxygen is thin; the hot days yield to frigid nights that could kill him.

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And every step of the way, Dad is sitting in front of holographic monitors, slowly bleeding out, “teaching” and leading his son.

Sophie Okonedo plays the mom who insisted her would-be Ranger boy go off with legendary Ranger dad for bonding and adventure. Zoë Kravitz — yes, she was born into the business, too — plays the boy’s sister.

A Will Smith action film that has him grimacing on his back, giving instructions, is nobody’s idea of a lot of fun. Jaden, a good-looking kid with a hint of charisma, has to carry the film and doesn’t have the presence to pull that off. And inexplicably, father and son have attempted Southern accents from over a thousand years in the future.

The digitally augmented scenery is striking and a couple of the action scenes work. But by focusing on Jaden, the movie feels a lot more “Karate Kid” than Will Smith action picture.

Since Shyamalan’s last film was the insipid “Last Airbender,” perhaps that’s the standard to judge it by — an undemanding, childish adventure picture for kids starring kids whose parents happen to be show-business folk.