Faux-metal bands in film are a tough joke to pull off;
This Is Spinal Tap set the bar way too high. Just ask anyone who saw Mark Wahlberg's unintentionally hilarious metal saga Rock Star. The Rocker (which probably should've been named The Drummer) is aiming for comedy, but it misses the Spinal Tap mark by a long shot.
Rainn Wilson plays Robert "Fish" Fishman, the drummer for Vesuvius, a power-pop metal band akin to 1980s Bon Jovi. When the rest of Vesuvius--played hilariously by Will Arnett, Bradley Cooper and Fred Armisen--decide to kick Fish out, his life takes a decidedly not-rock turn.
While the band stays wildly popular for the next 20 years, Fish is stuck in a dead-end office gig. His fate changes when his nephew Matt, played by Josh Gad (presumably because Jonah Hill was off being a funnier, chubbier, Jewisher kid somewhere else), needs a drummer for his band A.D.D. to play at the prom.
When A.D.D. (also composed of Teddy Geiger and Emma Stone) hits it big overnight because of a YouTube video leak (which includes a gratuitous shot of Wilson's ass) they hit the road. Christina Applegate and SNL's scene-stealing Jason Sudeikis also tag along as chaperone and manager, respectively.
The movie plays out predictably, all the way up to A.D.D. opening for Vesuvius at a huge show near the end of the film.
The Rocker finds a few hearty laughs along the way (even if it does stoop to plenty of sight gags) but the biggest crime this movie commits is the waste of huge comedy talent. When Jeff Garlin, Demetri Martin, Jane Krakowski, Aziz Ansari and Jane Lynch are underutilized, it's the same as having a rock band with no drummer.