Every few years a musical based on the work of an era or a band invades theaters. Think Mamma Mia or Across the Universe and you have the typical formula: Take some famous actors who can sort-of sing, and young leads who can actually belt out a tune, and then fill the space between the song medleys with a comedy-of-errors plot and you’re good to go.

Rock of Ages, a bland new addition to that musical tradition, rests entirely upon its Glee-like mash-ups. It’s a story about sex, drugs and rock and roll -- except without the drugs, no actual sex, and a very loose interpretation to what counts as rock and roll.

Based on the Broadway show, director Adam Shankman’s film begins with its wide-eyed ingénues, a Los Angeles-bound Oklahoma girl named Sherrie Christian (played by Julianne Hough) – yes, she’s named after the Journey and Night Ranger songs – and a hopeful rocker named Drew (Diego Boneta). After a chance encounter, smitten Drew gets Sherrie a job as a waitress at his workplace, the famously seedy L.A. rock venue The Bourbon, and the two share their hopes of becoming singers. But when Drew’s time to shine comes after the opening band for mega-star rocker Stacee Jaxx (Tom Cruise) drops out, misunderstandings and a taste of fame drive the two apart, setting the stage for their own VH1-esque spirals into depression.

There’s also a subplot about how The Bourbon is going under unless it can make $30,000, and another about how Stacee is being taken advantage of by his shady manager (Paul Giamatti) while falling in love with a slutty Rolling Stone reporter (Malin Akerman), not to mention the thread about the mayor’s wife (Catherine Zeta-Jones) trying to shut down venue in an effort to clean up the Sunset Strip. Throw in a kindly strip-club proprietor/fairy godmother for Sherrie (Mary J. Blige), and all you’re missing is a monkey and the kitchen sink. Just kidding! There is a monkey, played by Mickey the baboon.

To say the plot meanders is an understatement. It strolls from disparate plot point to disparate plot point without a care in the world. Chart the course of the movie and you’d end up with the dotted-line map left by the kids in Family Circus.

To its credit, Rock of Ages knows where its bread is buttered, and it’s not the cast or story; it’s the songs. The movie crams in as many tunes as it possibly can, with one number beginning only seconds after the previous one ends. The music is fairly entertaining -- after all, who doesn’t enjoy hearing Pat Benatar? Besides, if the songs stop, the movie will have to return to the plot, and no one wants that. Every minute not singing is a minute wasted, and Rock of Ages acknowledges this by doing its best to never let the characters say more than two lines before bursting into the next mash-up.

Unfortunately, the opportunity to wow audiences with these numbers is wasted, as Shankman and his choreographers have all the enthusiasm of sloths. Outside of one great pole-dance number that makes you realize just how athletic strippers are, the rest of the numbers are uninspired. Much of the movie is like watching a recording of a play: You can see how it would be fun if you were there in the audience, but all you’re getting is a two-dimensional rendition.

Part of the problem is the cast, which is split between those gamely playing along with the ridiculous set-up (like Zeta-Jones or Russell Brand) and those phoning it in (like Alec Baldwin and Giamatti). Because most of the cast isn’t known for singing and dancing, the bulk of the performing falls to Hough and Boneta -- and while they can certainly turn out a fantastic cover, the only one who can actually rock a song is Mary J. Blige, who nails every gritty note and is way too underutilized.

Cruise also gives a surprisingly good performance as Stacee Jaxx, who’s a combination of burned-out rocker and insane celebrity. While his singing voice is too pretty to be described as rock, Cruise really gets the self-loathing and doubt that assails his character. He’s also one of the few who really gets comedic timing, and as a result his gags are some of the few that actually succeed.

However, there’s a troubling sexist element to Rock of Ages in which every woman exists to shove her tongue down the throat of some male rocker -- unless she’s Sherrie, in which case she exists to be groped, pinched and stripped, even by her supposedly good-guy love interest. Men are either pathetic drunken slobs or pathetic wide-eyed dopes, and women just can’t get enough of them.

Yet none of the men seem to be enjoying this; there’s something weirdly PG about all of it, as if the film’s written by a group of 12-year-old boys trying to figure out what sex is. If it’s supposed to be a comment on sexism in the music industry, it misses the mark, and if it’s supposed to be titillating, then I feel sorry for the filmmakers’ significant others.

All of which is unfortunate, because the Sunset Strip in the late ‘80s is incredibly fertile ground for storytelling. While rock was dying an ignoble death in the rest of the country, choking on it own excess and collapsing under the weight of mainstream success, the Strip was the last bastion of the genre, a mecca for fans and a flashpoint for anyone who disagreed with them. While Cruise does his best to embody the decadences and bloat of the era, he’s the only one trying. Indeed, everyone else in Rock of Ages seems to be living in some magical fairy-tale land where rock is still the underdog and if you just sing with enough heart and soul you’ll make it big -- a narrative that would work perfectly if the movie were about pop music. But as a narrative for the Strip, it’s a mess, showing how profoundly the filmmakers do not understand the down-and-dirty history of rock and roll or its fans.

Rock of Ages opens Friday nationwide.