Jamie Dornan and Dakota Johnson star in the adaptation of the best-selling book. (Universal)

The movie adaptation of E.L. James’s best-selling book “Fifty Shades of Grey” was always destined to be a blockbuster; the quality of the film was beside the point.

After all, the erotic romance novel, based on saucy “Twilight” fan-fiction, did great business, despite being a 500-page lesson in how not to use a thesaurus. Millions of readers paid their dues, skimming countless boring scenes with a narrator who says nothing more profound than “holy cow!” and “double crap!” so they could get to the good stuff: bondage-laced sex scenes between the story’s innocent protagonist and her impossibly hot, impossibly rich damaged-goods love interest.

So you have to hand it to director Sam Taylor-Johnson and screenwriter Kelly Marcel for making a film that, while not as titillating as the book, is a much better adaptation than the source material deserves.

The pair stripped out the more idiotic dialogue — leaving behind one “holy cow” —streamlined the preposterous plot and, most important, made a very smart casting decision by placing Dakota Johnson, the daughter of Melanie Griffith and Don Johnson, in the lead role.

Between pratfalls and perfectly timed facial expressions, Johnson brings a steady stream of humor to the part of college senior Anastasia Steele, while also nailing the sexy ingenue vibe. It’s immediately clear that Johnson is up to the task of transforming an annoying, even embarrassing character into a likable one: When Anastasia first enters the Seattle offices of billionaire Christian Grey, she looks around in wonder at the tall, impeccably dressed blondes that work for him, before glancing down at her own getup with a hilarious look of dismay.

Casting Dakota Johnson as the lead was a smart decision. (Universal Pictures/Focus Features)

The slightly unkempt Anastasia is there to interview the 27-year-old self-serious entrepreneur for the school newspaper. In both the movie and the book, the awkward first meeting begins with Anastasia taking a spill as soon as she enters the office. From there, the novel quickly becomes an overwrought description of funny feelings down below, while the movie wisely keeps things in comedic territory for as long as possible.

Comedy and chemistry are not mutually exclusive, but during this opening scene of supposed animal magnetism, the spark doesn’t quite materialize. Irish actor Jamie Dornan portrays the wounded moneymaker with such restraint that he never channels anything more than his own good looks (which are indeed very good). In any case, so begins a flirtation between the two that ultimately leads to a nondisclosure agreement — never a good sign during a relationship’s early stages — followed by an introduction to Christian’s “playroom.” Or, as Anastasia calls it, his Red Room of Pain: a locked chamber in Christian’s penthouse where he likes to get busy with the help of various implements — floggers, whips, shackles, you know, the usual.

All of which leads to yet another contract. Will the virginal Anastasia agree to be the submissive to Christian’s dom? He’s upfront about the fact that he doesn’t “do romance,” but he likes her so much, he’s willing to sweeten the deal. If she spends each weekend being, essentially, his sex slave, he’ll spend one night a week doing normal couple things. Two days of punishment in exchange for dinner and a movie seems like a raw deal in every sense, and Johnson makes this whole ridiculous scenario seem believable by responding not with impassioned earnestness but with a healthy dose of incredulity: “What do I get out of this?”

That contract is about as much plot as you’re going to get. Whether she will sign on the dotted line is the mystery that powers the narrative, and it isn’t much to go on. Of course, the tedium of doing business is broken up by about 15 minutes of sexcapades. Compared to the book, which features a much-discussed scene involving a tampon, the movie is a model of moderation. Every sexual encounter plays out to the soothing strains of some lovely vocalist (Sia, Beyoncé), and careful framing means we see plenty of skin, but not as much as you might expect for a chronicle of fringy sexual habits. Dornan doesn’t even get totally naked for the camera.

Taylor-Johnson clearly was going for an R rating, and even with what the MPAA deemed “unusual behavior” — including one difficult-to-watch whipping — no scene comes close to earning an NC-17 designation.

In the end, there’s nothing here we haven’t seen before. But there’s also nothing as agonizingly awkward as James’s prose.

Fifty Shades of Grey

★ ★

(125 minutes, at area theaters) is rated R for strong sexual content including dialogue, some unusual behavior and graphic nudity, and for language.