There’s a difference between dumb-slash-funny and merely dumb, and “Zoolander 2” falls into the second category while missing the first entirely. After gestating for nearly fifteen years—far longer than the wait from “Revenge of the Sith” to “The Force Awakens”—this lumbering, laugh-free sequel to the 2001 farce about a rock-stupid male fashion model does for Ben Stiller’s reputation what “The Love Guru” did for Mike Myers’. One of the few bright lines in it comes when a character remarks that fashion models and pop music stars are genetically almost the same, except that the former lack two genes, those for intelligence and talent. By the end of the movie you begin to suspect that the filmmakers share their genetic makeup with the models.

As it happens, “Zoolander 2” opens with one of its few clever bits, one that focuses on a music star—Justin Bieber, who, in just the first of many cameos scattered throughout the movie, is chased through the streets of Rome by a motorcyclist wielding an automatic and finally gunned down in a ludicrously prolonged hail of gunfire. Before expiring he manages—after seeing to it that his hair is properly arranged—to send off a selfie to his Instagram followers, adopting in it one of the pouty poses of erstwhile superstar model Derek Zoolander. That’s a clue for Interpol Fashion Crimes investigator Valentina Valencia (Penelope Cruz), who’s trying to discover the villain killing off the world’s most beautiful pop stars, with Bieber only the most recent victim.

But Zoolander has disappeared after losing his wife in the collapse of the headquarters of his pet project, the Center for Kids Who Can’t Read Good and Wanna Learn to Do Other Stuff Good (the sort of overkill that continues into the rest of the movie), and then having their young son removed by CPS because of his incompetence as a father. He’s been living as a recluse in the snow-covered wilds of northern New Jersey, but Billy Zane arrives with a message from fashion impresario Alexanya Atoz (Kristen Wiig, almost unrecognizable in outlandish getups and talking in a mush-mouthed style that squashes all the consonants) inviting him to participate in her famous Incrediball. He decides to go, as does his onetime rival Hansel (Owen Wilson), who—after suffering a supposedly devastating facial injury—has taken to living in the sand-covered wastes of Malibu, along with a group of weird companions) and receives a similar invitation.

What follows is a film that’s elaborately put together, with flamboyant production design by Jeff Mann, art direction by Saverio Sammali and Armando Savola, sets by Lucy Eyre and costumes by Leesa Evans, all photographed colorfully by Dan Mindel in sites that include some attractive Italian locations. But it’s also an incoherent jumble of ridiculous plot twists that bring the two dunces, who overcome their initial hostility to form a team with Valentina to crack the case.

The rambling, numbingly unfunny scenario concocted by Stiller and three other scripters involves the guys’ humiliation at the hands of Alexanya and her supremely irritating amanuensis, a grubby hipster called Don Atari (Kyle Mooney), the discovery of Derek’s long-missing son Derek, Jr. (Cyrus Arnold), and the involvement of the chubby boy in a plot masterminded by Zoolander’s nemesis, the evil Mugatu (Will Ferrell), whom Zoolander idiotically helps to escape from Fashion Prison, that has something to do with a creation myth featuring the first male model (called Steve), the fountain of youth and the assassination of all those music stars. Along the way many more misguided cameos occur, from the likes of Benedict Cumberbatch (as an androgynous model called All), Susan Sarandon, Neil deGrasse Tyson, Willie Nelson, Katy Perry and scads of real-life fashion designers, who on the basis of their appearances here, would be better off avoiding the camera. There’s a big final face-off between Zoolander and Mugatu featuring knives, a bomb, and a volcano apparently roiling right under the Eternal City. Despite all those elements it fizzles like a defective firecracker.

Of the stars Wilson’s dazed-and-confused persona fares best, while Stiller’s doofus shtick and strained malapropisms quickly cease to be at all amusing and Cruz has little to do but appear voluptuous. As is often the case, Ferrell’s over-the-top turn comes off as more creepy than funny, and one suspects that much of his material is misbegotten improvisation. But certainly the most horrendous performance comes from Mooney, although the writing bears greater culpability than the actor. Of the cameos, Bieber’s is easily the best, though Sting comes through at the end with a “Star Wars”-themed appearance that includes that line about genetics.

Donald Trump did a blink-and-you’ll-miss-it cameo in the first “Zoolander.” His absence here represents a wiser choice than his skipping the last Iowa candidates’ debate. Even smarter would be your decision to ignore “Zoolander 2.”