About halfway through “Sahara,” the character played by William H. Macy receives some surprising news and declares, “That’s not possible.” Faced with another unlikely scenario toward the end, Steve Zahn announces, “It’ll never work.” Regrettably, most moviegoers will already have come to similar conclusions about the film itself, a slapdash affair that fails to clear even the low hurdles of plausibility that prevail in the action genre.

Based on the novel by Clive Cussler, “Sahara” follows the adventures of Dirk Pitt and his colleagues at the National Underwater and Marine Agency, a kind of nautical salvage outfit. What are underwater adventurers doing in a movie entitled “Sahara,” you ask? It seems that following its disappearance in 1865, a Civil War ironclad motored across the Atlantic to Africa and thence up the Niger River. How exactly an armored steamboat not designed for ocean travel managed such a voyage, and why, are questions “Sahara” does not bother to answer. It’s just there, okay?

Dirk (played by Matthew McConaughey in a shameless bid for a franchise role) has long been obsessed with rumors of the African ironclad, so when a coin connected to the ship shows up in Mali, he and his NUMA buddy Al Giordino (Steve Zahn, looking not the slightest bit Italian) go to investigate. Along the way, they meet a pair of doctors from the World Health Organization – a beautiful, headstrong Spanish lass (Penelope Cruz) and a decent, avuncular African-American (Glynn Turman) – one of whom is brutally killed shortly thereafter. (I’ll let you guess which.) The doctors believe Mali to be the epicenter of a strange and lethal new plague that is spreading across West Africa, and so join the NUMA boys on their quest.

From there we are treated to a litany of stock situations (the speedboat chase, the trek across the desert on foot, the last-second bomb defusing) and characters (the African warlord, the kindhearted rebel, the sleazy French industrialist) occasionally interspersed with developments so comically ludicrous that they can’t help but be original. The plague, for instance, is caused by toxic waste from a space-age, solar-powered incinerator in the desert which, despite being about a mile wide, is apparently unknown to the outside world. The whereabouts of the ironclad – right next to the incinerator, of course! – are discovered when Al, playing a game of soccer with Mali kids, chases a lost ball down the street and into a hidden grotto, where it conveniently rolls to a stop right below a primitive cave painting of the ship. In the grand finale, Dirk and Al take on the warlord’s helicopter gunship using the iron-clad’s 150-year-old cannon. And so on.

At least four separate screenwriters worked on the script (not including Mr. Cussler), and it shows. Dirk finds historical documents describing the ironclad as a “death ship” that spread some mysterious pestilence when it arrived in Africa, clearly insinuating that it’s causing the current plague. But, according to what we later learn, it’s not. The plague is also somehow connected (though we’re never told how) to a red algae in the river that, if it makes it to the ocean, will spread across the globe. But this doomsday scenario is sketched so vaguely that the movie quickly drops it altogether.

The entire mess is directed by Breck Eisner, son of outgoing Disney CEO Michael Eisner. (Yes, apparently one of the most powerful men in Hollywood named his son after shampoo.) Young Eisner does little to allay the inevitable suspicions of nepotism. Although he keeps the film moving forward at breakneck pace – the characters zoom around by boat, camel, vintage car, and improvised dune racer – it never really goes anywhere. Mr. Eisner’s tendency is to compensate for his lack of clear storyline by simply turning up the volume and bludgeoning us into acquiescence. “Sahara” isn’t merely loud, it’s loud in a sloppy, playing-with-your-folks’-stereo-when-they’re-out-of-town way. Classic rock tunes (“We’re an American Band,” “Magic Carpet Ride”) rise deafeningly at the drop of a hat. When Dirk and an assassin engage in fisticuffs early in the movie, each blow sounds like a collision between tectonic plates.

Ms. Cruz and Mr. Zahn adequately fulfill their respective functions of comeliness and comic relief (though the latter bears little resemblance to the muscle-bound sidekick of Mr. Cussler’s novels), and Lambert Wilson manages several good Merovingian sneers as the evil industrialist. (It is either a coincidence or a rather dull inside joke that this is the second movie entitled “Sahara” Mr. Wilson has starred in, the first being a 1983 Brooke Shields vehicle.)

But carrying the movie is Mr. McConaughey’s burden, and he can’t do it. Slathered in fake tan, he’s about as orange as any human being since Al Gore in the 2000 presidential debates. But apart from this unnatural hue, nothing else about his character registers very clearly. Is Dirk Pitt supposed to be a straight arrow who obeys orders? A rebel who goes his own way? A tough guy? A sensitive guy? A ladies’ man? “Sahara” offers bits of evidence for all these characterizations, but not enough to make any one of them stick.

The model the filmmakers are following is clearly the Bond franchise, which has been sketching its hero rather haphazardly for decades now. But James Bond doesn’t need to cohere as a character anymore, because he’s already familiar to anyone not living in an underground bunker. Dirk Pitt, by contrast, is a new flavor for moviegoers. Though he’s appeared in countless Cussler books, only one other has made it to the big screen, the 1980 bomb “Raise the Titanic.” The most glaring failure of “Sahara” may be that it doesn’t help its audience get to know Mr. Cussler’s hero – or give them much reason why they’d want to.