After more than twenty years somebody decided that we needed a sequel to “Jumanji,” Joe Johnston’s overstuffed 1995 special effects orgy about a board game come to life that put the late Robin Williams into manic overdrive. The result of this uninspired impulse is a script, cobbled together by no fewer than four writers, that’s an unwieldy mash-up of a 1940s-style cliffhanger serial and “The Breakfast Club,” peppered with the slightly naughty humor obligatory in PG family fare nowadays. It boasts, however, a game cast that gives the material more than it’s worth, and for parents and kids starved for holiday entertainment it may prove a plausible alternative to a second helping of “The Last Jedi.”

The premise is that four dissimilar high schoolers, locked up together in detention and ordered to clean up the school basement, happen up the game (now an old video model) and decide to play it. Suddenly the find themselves in a jungle, transformed into the very different avatars they chose. Geeky gamer Spencer (Alex Wolff) becomes Dr. Smolder Bravestone (Dwayne Johnson), an Indiana Jones-style explorer; hunky football star “Fridge” Johnson (Ser’Darius Blain), who’s angry with Spencer because a teacher has identified the geek as the writer of one of his papers (threatening his football eligibility), morphs into short, frenetic zoologist Franklin Finbar (Kevin Hart), Smolder’s sidekick-flunky; self-absorbed cheerleader Bethany Walker (Madison Iseman) is remade as Sheldon Oberon (Jack Black), an anxious professor of cartography and paleontology; and shy, bookish Martha Kaply (Morgan Turner) is turned into Ruby Roundhouse (Karen Gillan), an athletic powerhouse. Fleeing rampaging beasts, they meet Nigel (Rhys Darby), a programmed instruction book who informs them that they are now trapped in Jumanji, and that to complete the game and return to their world, they must recover a gem removed by explorer John Van Pelt (Bobby Cannavale) from the statue of a jaguar and return it to its rightful place, saving the realm from his baleful influence.

But Van Pelt has no intention of giving up the gem, which grants him power over the forest’s animals, and sends his army of critters and (oddly enough) motorcycle thugs to deal with the interlopers, each of whom has three lives to lose before they actually die. As they follow a series of clues that direct them toward their destination, they encounter Alex Vreeke, who as a boy (Mason Gussione) was trapped in the game in 1996 and has survived since then in Jumanji as pilot avatar Jeff McDonough (Nick Jonas). He joins them in their quest, which proceeds through an avalanche of battles, escapes, dangers (and assorted death scenes and rebirths) until they succeed. (That can hardly count as a spoiler; after all, this is meant as family holiday entertainment.) A postscript designed to elicit tears and sniffles wraps things up.

If all this sounds too confusing by half, not to worry; none of the details matter much. The whole raison d’étre of “Welcome to the Jungle” is random CGI spectacle leavened by what’s meant to pass for humor; think of the “Night at the Museum” movies ratcheted up a notch, spiced up by such ironies as a geek turned into a brawny (but still scared) hero, a hunk morphed into a shrimp, a mousy bookworm transformed into a sinewy babe, and a babe learning to accept herself as an overweight guy (obsessed, of course, with “her” first experience of the male physique—the basis of many puerile jokes). In the process, of course, each of the youngsters learns sage lessons about him or herself and the importance of empathy and concern for others.

What modest pleasures the movie affords come, despite a few irritatingly phony moments, from the effects—youngsters may especially appreciate the icky insects crawling over people’s faces and the snakes, in one instance approximating a famous moment from “Indiana Jones”—and the ready-to-do-anything cast. Johnson again exhibits a nice self-deprecating touch, and he and Hart demonstrate an effective screen rapport for a second time after “Central Intelligence.” Black does his effeminate routine with gusto, Gillan pulls off her sultry one with similar enthusiasm, and Jonas manages to bring some pathos to the table, while the characters’ younger selves are decently managed down the line, with Wolff especially effective. Only the snarling Cannavale embarrasses himself, forced to play one dull note. Director Jake Kasdan choreographs his first jumbo-sized studio project well enough, though the picture shows few of the quirky asides his smaller efforts have. The technical credits (Owen Paterson’s production design, Laura Jean Shannon’s costumes, Gyula Pados’ cinematography) are fine down the line, but the editing by Steve Edwards and Mark Helfrich might have trimmed a little from the two-hour running-time, and Henry Jackman’s score is very loud.

Like most such games, “Jumanji” probably should have been left to collect dust in the closet or attic, but for viewers content with second-hand merchandise “Welcome to the Jungle” may suffice as brainless, ephemeral fun.