But in one of the pleasant surprises of the summer-movie season, this genre mash-up is lean, handsomely crafted entertainment. It works precisely because it acts as if there’s no joke. You’ve got cowboys, and you’ve got aliens. ’Nuff said.

It certainly starts with a bang. A man (Daniel Craig, “Quantum of Solace”) wakes up in the middle of nowhere. He has a curious-looking metallic cuff shackled on his left wrist, but damned if he knows how he got it. In fact, he can’t remember anything, not even — in a fitting homage to Clint Eastwood’s spaghetti-Western heroes — his own name. But this Man with No Name hasn’t lost his ability to kick ass, 1870s-style, when he is suddenly surrounded by a trio of menacing ZZ Top look-alikes. And we’re off to the camptown races. The manacled cowpoke wanders to the nearby town of Absolution; has a run-in with a spoiled, trigger-happy punk (Paul Dano, “Knight and Day”); and is subsequently arrested after being identified as a stagecoach thief named Jake Lonergan. Our taciturn hero hardly has time to catch his breath before tangling with the punk’s father, feared cattle baron Col. Dolarhyde (Harrison Ford, “Morning Glory”).

But there are bigger threats: Flying saucers are roaming the countryside, blasting buildings to smithereens and stealing off with townsfolk.

This is familiar stuff for Western fans. Loosely based on a graphic novel of the same name, the movie dutifully lets Old West archetypes fill in for character development. The tactic allows the narrative to gallop swiftly to the inevitable genre mash-up.

It’s a minor miracle that a concept so fraught with silliness is treated straightforwardly, and with so much success. Director Jon Favreau (“Iron Man 2”) and a gaggle of screenwriters led by Roberto Orci and Alex Kurtzman (“Star Trek”) don’t wink at their audience. The filmmakers understand the mythological power of the Western, and they similarly appreciate its flip side: the deeply rooted fear and xenophobia that gave rise to spaceinvader fantasies. The connective tissue linking the genres might be tenuous — perhaps why Hollywood hasn’t really done it before — but they both deserve reverence, and “Cowboys” is reverent about its B-movie origins.

The actors follow suit. Craig is pitch-perfect as the strong silent type. Ford flirts with self-parody — he glowers so much you half-expect his jaw to dislodge and pop off — but he reveals glimmers of heart beneath the irascible facade. And with actors like Sam Rockwell (“Conviction”) and Olivia Wilde (“TRON: Legacy”) rounding out the cast, it’s tough to go wrong.

Which might be why “Cowboys & Aliens” doesn’t go wrong.