Starring Zac Efron. Rated 14A. Now playing.

There’s a lot of controversy these days about the lack of diverse casting in movies. If nothing else, suffering through Robert De Niro in Dirty Grandpa proves another sad fact about Hollywood: there are way too many roles for shameless old white guys who’ll do anything for big bucks.

De Niro stars as horny, foul-mouthed senior Dick Kelly, who decides to go on a Spring Break road trip with his strait-laced grandson (Zac Efron). Dick has recently lost his wife to cancer. His remedy for grief? Getting laid by someone young enough to be his granddaughter. Of course, he has a more noble purpose in mind for his uptight grandson. He wants to teach him to loosen up by employing a strict regimen of sex, drugs, and bare-assed humiliation.

In case we’re still fuzzy on the creative thrust here, director Dan Mazer lets us know just what kind of film this is going to be early on. That’s when we see Dick furiously masturbating to porn as his grandson inadvertently interrupts. Efron, who wears the same vacant expression from start to finish, can’t quite muster up enough ability to look genuinely disgusted. But if De Niro can shift to cruise control—and stay there—what’s the difference?

This is one grubby little movie. But the shabbiest aspect of the whole enterprise is that the script can’t even fully embrace its truly sordid nature. The story tries to have things both ways, injecting the kind of stale sentiment that’s meant to redeem all the degradation we’re forced to witness. It’s early yet, but Dirty Grandpa has my heartfelt vote for worst movie of the year.