The diminutive yellow gibberish-spouting critters that provided the support staff for Gru in the “Despicable Me” movies take center stage in “Minions.” It’s a ploy that didn’t work terribly well for the “Penguins of Madagascar,” and fares even worse in this case, though young kids may enjoy the hullabaloo of what amounts to a manic but rarely inspired prequel—cinematic hamburger rather than filet minion.

Parents will probably be most amused by the prologue, which follows the development of the minions from amoeba status to the near-present, with Geoffrey Rush providing helpful narration about how their drive to serve the most despicable villain they can find combines with their overeager ineptitude to cause the downfall of a succession of bosses—the T Rex, a muscular caveman, an Egyptian pharaoh, Dracula and finally Napoleon—before they find themselves in exile in an Arctic cave. Gloomy over the loss of somebody mean to serve, they fall into despondency, until taller-than-most Kevin hatches a plot to go out into the world and locate a new master. In response to his request for volunteers, he’s joined by Cyclops-eyed, ukulele-playing Stuart and—to Kevin’s chagrin—Bob, who’s even shorter and more childish than most of the brethren. To the cheers of their hordes of babbling compatriots, they’re soon off to find some dark wizard to serve.

After a stop in the New York City of 1968 (where an election sign for Richard Nixon leaves one briefly hoping that he’ll become The One they choose), the guys happen upon a television announcement for Villain Con in Orlando and hitch a ride there with what turns out to be a bad-ass family of bank robbers (featuring Michael Keaton and Allison Janney as Walter and Madge Nelson). After auditioning a few of the attendees, they settle on the event’s Queen Bee—sultry Scarlet Overkill (Sandra Bullock), who takes them back to her London castle where she plots with her inventor/partner Herb (Jon Hamm) to purloin Queen Elizabeth’s crown—thus fulfilling the childhood dream of being a princess she shares with probably every little girl in the audience.

It’s at this point that “Minions” shifts into the default mode among second-rate animated family fare nowadays—the big, action-packed final confrontation, which as usual seems to go on forever. In this case after the minions’ attempt to steal the crown go awry, the vengeful Overkill targets her supposed henchmen when Bob is appointed the new king of England, employing rocket-propelled bombs among other nifty weaponry against them. In the process Kevin grows to the size of Godzilla and does battle with her. If all that sounds like bad narrative news, it is, particularly because the whole business drags on so interminably, though it’s periodically interrupted by the sight of the minions that have been left behind in the Arctic making their way over various continents to reach England—brief episodes that are often more amusing than the central plot.

It’s all made worse by the fact that Scarlet never becomes much of a character—she’s certainly no Cruella De Vil—and Bullock doesn’t manage to give her much panache beyond a generic sort of shrillness. Nor does Hamm bring a great deal of punch to the swinging-sixties hipster figure of Herb. It’s not the fault of the actors; the script provides very little for them to work with, any more than it does for Keaton and Janney. And that’s ultimately where the weakness of “Minion” lies—in the nondescript human characters (including Jennifer Saunders’ Queen Elizabeth) and dialogue for them that lacks sharpness and tang.

That leaves most of the burden for the minions to carry, and for a while they’re enough, with their cheerful nattering (all provided by Pierre Coffin) and expressive faces doing the work. Ninety minutes is a long time for that to suffice, however, partially in the later reels, and it’s a relief when Gru (again voiced by Steve Carell) shows up at the close to become the little critter’s new boss. The scenes featuring him in the closing credits are happier than most of what’s gone on before. There’s also some mild amusement to be found in Heitor Pereira’s score, which uses snippets of old-time favorites (at one point the minions do a gabbled version of the Monkees’ theme song) as well as classical pieces (Elgar, for instance).

Perhaps that bodes well for “Despicable Me 3,” which is already in the pipeline, though the quality drop-off from the original installment to the disappointing sequel was pretty severe. But “Minions,” while visually fine, only proves that spin-offs for supporting characters are generally a bad idea, even if—as seems likely in this case—they may prove box-office bonanzas and spawn sequels themselves. The picture also confirms the continuing gap between Pixar product and the efforts of virtually everybody else in the animation business. The former’s success in merging heart and humor isn’t being matched elsewhere, but the hunger for family entertainment is such that even second-rate stuff like this continues to prosper.