Much of what happens in “Sahara” relies on the heroes finding very old items that are still in working condition, including the wreckage of an airplane and a 150-year-old cannon. If these machines did not function — which they surely would not if you or I were to find them — there would be no movie. And if there were no movie, then there would have been only eight films called “Sahara” in the past hundred years, rather than nine.

It is not just in its blank title that the new “Sahara” feels overly familiar. Its treasure-hunter protagonists are wise-cracking and resourceful; its snarling villains have British accents; its action sequences are competent but no better than an average episode of “Alias”; its plot twists stretch the boundaries of how much “coincidence” viewers can believe. It seems less like a movie and more like excised pages from an Indiana Jones script being enacted by B-listers.

It is set in Nigeria and environs, where two American thrill-seekers and hack historians — the handsome and cavalier Dirk Pitt (Matthew McConaughey) and his less-handsome but more-cavalier companion Al Giordino (Steve Zahn) — are working with Ret. Adm. James Sandecker (William H. Macy) to recover an ancient tomb currently lying at the bottom of the sea. While in town, Dirk and Al pursue leads on one of their favorite obsessions: the mysterious 1865 disappearance of a Civil War ironclad ship. Legend says it was sighted off the coast of Africa, a theory discounted by most historians because ironclads weren’t meant to cross the ocean. But Dirk and Al find some solid evidence that the warship did, in fact, make an appearance near the Dark Continent, and they convince Sandecker to let them take his boat up the river to look into it.

Meanwhile, the World Health Organization has sent its hottest, most incomprehensible doctor, Eva Rojas (played with hot incomprehensibility by Penelope Cruz) to Western Africa to investigate a series of unusual illnesses that might point to a larger plague. But Mali is in the midst of its own civil war (no ironclad ships, though), with half the country being run by a warlord named Kazim (Lennie James) who, along with snotty English businessman Yves Massard (Lambert Wilson), is trying to keep the W.H.O. out of Mali. Why? Oh, but the reasons are nefarious indeed.

Dirk, Al and Eva all wind up together, two Hans and a Leia, embarking on various “Star Wars”-esque missions. At one point it looks as though they will be attacked by Jawas, and when they infiltrate a secret factory, it is reminiscent of sneaking aboard the Death Star. Zahn is wry, as always; McConaughey grins a lot, as always; and I can’t understand a word Cruz says, as always.

Directed by Breck Eisner (out-going Disney chief Michael Eisner’s son) and adapted from Clive Cussler’s novel by four, count ’em four, screenwriters, “Sahara” gets crazier and crazier as it goes, with the heroes outrunning bullets, resurrecting ancient airplanes, and stumbling across great treasures at the most extraordinarily useful times. A little inventiveness is one thing, but this movie strains credibility at every turn.

In addition, it has far too much exposition before anything really happens — and even once it does happen, it’s the same old thing. An attempt has been made to re-create the jaunty thrills of “Raiders of the Lost Ark,” but it doesn’t even come close. These characters are jokey and generic, and their adventures are common.

C+ (2 hrs., 4 min.; PG-13, a lot of action violence, none of it too graphic; a couple very mild profanities.)