Oblivion is derivative enough that it broaches the border between loving tribute to and total ripoff of a handful of science-fiction/post-apocalyptic classics.

Apparently, that was the point, if you listen to director Joseph Kosinski, who also wrote the graphic novel that served as the source material for this wannabe blockbuster. Thanks to slick, new-feeling special effects and concepts and strong performances by Tom Cruise and Andrea Riseborough, it never quite crosses over to the dark side And so what you're left with is a perfectly good, but never really great popcorn film that is a decent way to spend a Saturday night in but can't sniff the classics that inspired so many of its elements.

Cruise and Riseborough play Jack Harper and Vika Olsen, respectively, the last technicians remaining on Earth after it was nearly destroyed in an alien invasion decades earlier. Jack and Vika live far above the surface of the planet. They spend their days repairing drones that protect massive power stations harvesting seawater that, through fusion, powers Tet, a space station to which they will soon return. They spend their nights playing house, with only Jack's vague memories of the Empire State Building pre-invasion and Sally (Melissa Leo), a supervisor stationed on Tet, disrupting their lonely existence.

That is, it's a lonely existence until Jack stumbles across the crew from a NASA spaceship and manages to save one of its members Julia (Olga Kurylenko) from annihilation at the hands of the "Scavs" -- the term for the few aliens who attacked Earth some 60 years ago and that have managed to remain on the surface.

Julia is, quite literally, the woman of Jack's dreams. He's been seeing her in those memories that sporadically interrupt his pleasant life with Vika, and her arrival throws everything he and his companion think they know into doubt.

There's a pervasive sense that nothing is as it seems from the very start of the film. Jack and Vika have both had their memories wiped for "security purposes." They are promised a place on Titan -- one of Saturn's moons -- with the rest of the survivors once their work is done. And yet their only human contact, outside of each other, comes through Sally, who, despite having Melissa Leo's face and voice sounds an awful lot like a female HAL as she repeatedly asks Vika if she and Jack are "still an effective team." Surely someone should fill in for her on Tet one of these days?

The Scavs, meanwhile, even in their weakened state hardly seem like the type of aliens that could have caused a full-scale evacuation of the planet.

It's hardly a shock when it turns that nothing, in fact, is as it seems -- when Morgan Freeman and Jaime Lannister (aka Nikolaj Coster-Waldau) turn out to be among the hated Scavs.

Once all is revealed, Cruise, looking quite a bit better than he did in Mission Impossible: Ghost Protocol, is shuttled through the all-too-familiar action-oriented paces that every third act in a film like this seems required to have these days. It's entertaining enough, but it still feels like a letdown relative to the film's more meditative moments.

Oblivion is never more interesting than when Cruise is catnapping on the surface at a camp with a basketball hoop, books and hundreds of LPs. Had it been able to maintain that level of intrigue throughout, this film might have been something more.