Jim Henson’s stable of wacky puppet characters managed a satisfyingly silly reboot in 2011 with “The Muppets,” but this sequel represents a rather swift decline in their fortunes. The opening musical number of “Muppets Most Wanted,” entitled “We’re Doing a Sequel,” contains a line saying that follow-ups are never as good as the original. Then the movie goes on to prove how true the sentiment is.

To be fair, that song-and-dance sequence, the first of many such uninspired numbers, does contain one brilliantly funny moment. Among discarded suggestions for what the sequel might be—lame gags like “Gonzo With the Wind” or a simple repeat of the previous movie—is one opting for an existential reverie on the human condition, complete with a black-and-white glimpse of the Swedish chef and a scythe-wielding Grim Reaper at a chessboard. It’s a witty, sophisticated allusion to Bergman’s “Seventh Seal.”

Unfortunately, that pattern becomes characteristic of the whole movie. There are gems to be found here—bits of dialogue and the occasional sight gag that will bring a smile, if not a belly laugh. But they’re relatively rare, embedded in a picture that by and large is pretty mediocre, likable enough but totally ephemeral, dependent as it is on a string of cameos from guest stars, some of whom are identified but most of whom are left to the vagaries of audience identification. This must be the largest collection of blink-and-you’ll-miss-them moments in a movie since Mike Todd’s “Around the World in 80 Days” (which boasted more than forty of them), but in the majority of cases the personalities aren’t given anything funny to do; the jolt of recognition is supposed to be enough. (It isn’t.)

As to plot, it’s a hokey kidnap-and-replacement business with a heist motif. After the success of their reunion movie, the Muppets are at a loss as to what to do next. Enter Dominic Badguy (Ricky Gervais), super-promoter, who proposes a world tour. But the stops are designed merely to provide cover for a series of next-door thefts that are way stations that will culminate in the theft of the Crown Jewels from the Tower of London under cover of a wedding that finally uniting Kermit and Miss Piggy. But it won’t be the real Kermit: he’s been kidnapped and replaced by Constantine, the self-described world’s most dangerous frog and Dominic’s boss, who’s escaped from a Siberian gulag prison presided over by Nadja (Tina Fey). He’s a dead ringer for Kermit, except for a prominent birthmark and his thick Slavic accent; and the elaborate ceremony is merely a device to get him and Dominic close to their target.

While the tour continues—allowing for a succession of comic numbers before crowds Dominic has bought and paid for—the script repeatedly returns to Kermit’s prison, where Nadja becomes enamored of him and insists that he oversee the camp’s annual prisoners’ show, in which Jemaine Clement and Danny Trejo are among the most prominent performers (though Josh Groban brings the most powerful voice). Meanwhile a French Interpol detective called Jean-Pierre Napoleon (Ty Burrell) and CIA agent Sam the Eagle do an odd-couple team-up as cops trying the track down the perpetrators of the heists that, strangely enough, coincide with each leg of the Muppets’ tour in Berlin, Madrid and Dublin.

Part of the problem with “Muppets Most Wanted” lies in its repetitiveness—the various plot threads are juggled for variety, but within each the gags (the other Muppets’ obliviousness to the fact that Constantine isn’t Kermit, the love of Nadja and the prisoners for Broadway-style song-and-dance numbers, the one-upsmanship between Napoleon and Sam) gets tiresome. Moreover, the juxtaposition of the actors with the foam-and-fluff Henson creations proves only erratically successful. Even during the run of “The Muppet Show,” it was always a hit-and-miss proposition as to whether the combination of guest star and puppets would gel; sometimes it did, but in other cases it fell flat. One might think that Gervais and Fey would fit comfortably into Muppet World, but in fact they seem uncomfortable in it, both trying too hard. It certainly doesn’t help that the musical numbers in which they’re expected to shine are among the worst in the bland lot provided by Bret McKenzie. Burrell works better—for one thing he avoids having to sing a song, which is probably a blessing. But he’s reduced to doing little more than an extended Inspector Clouseau routine in which the big joke is the French penchant for short workdays and extended vacations. What a scream.

As might be expected, all the plot complications turn out for the best, with the bad guys consigned to police care and Henson’s merry band restored to their usual familial bonds. And “Muppets Most Wanted” looks fine, with expert puppetry all around, while director James Bobin (who wrote the script with Nicholas Stoller) keeps things moving at a reasonable clip, even though the narrative convolutions (and James Thomas’ editing) stretch it out to an overlong 106 minutes.

In the end, though, after the charm of the 2011 reboot, this winds up as a genial but barely average installment in the long line of Muppet movies, on the level of the nineties efforts that brought the series to an halt for more than a decade. It’s preceded in theatrical release by a “Monsters University” short, “Party Center,” which mirrors the feature in being one of Pixar’s less memorable pieces, too.