Right back to The Muppet Show, Jim Henson's lovable puppets have relied on celebrity foils to bring out their best. There's just something about an absurdly famous person talking to a fuzzy, few-feet-tall puppet as a peer. But as Muppets Most Wanted demonstrates, there's a dangerous line being toed for that formula to work -- and it's closer than it has often seemed.

Muppets Most Wanted attempts to build on the unbridled success of 2011 reboot The Muppets, which was the best reminder since The Muppet Christmas Carol of how heartwarming and simultaneously subversively funny Kermit and company can be. It worked because Jason Segel and Amy Adams played their parts with wide-eyed optimism and sincerity, and because they were all too willing to get out of the way of the true stars of any film with the Muppets -- the Muppets themselves, of course.

Most Wanted picks up right where its predecessor left off, in terms of plot and plot alone. Immediately following the show that saved Muppets Studio, the brood plots a world tour with the help of a nefarious promoter by the name of Dominic Badguy -- "it's pronounced bad-ghee." Badguy, played by Ricky Gervais, is in cahoots with Constantine, a criminal genius that just so happens to look exactly like Kermit, save a signature black mole above his mouth.

Just as Badguy works his way in with the Muppets -- over the objections of Kermit -- so is Constantine escaping from a high-security Siberian prison. Not long after, Badguy's intentions become clear, and he engineers an identity swap, gluing a mole on to Kermit that sends him back to the Siberian prison Constantine escaped and applying the right shade of green makeup to Constantine's mole to disguise him as the leader of the Muppets.

Much of the first act and all of the second sputter about as the film tries to find a rhythm and an identity. Badguy and Constantine commit a series of capers in coincidence with the Muppets' European tour -- setting Sam the Eagle and Interpol agent Jean Pierre Napoleon (Ty Burrell) on their tail. Kermit, meanwhile, despairs when it appears that his friends are not coming to the rescue (indeed, they don't even notice that he's gone), then forms a bond with Nadya, a Russian prison guard played by Tina Fey who has a deeply burning crush on the frog.

At its heart, this is a story about how Kermit's leadership is underappreciated -- easily undermined by someone who lets the Muppets do whatever they want and tells them what they want (rather than need) to hear. Even Miss Piggy, about to be married to the frog she thinks is Kermit, comes to realize that the frog isn't always greener on the other side.

There's a bit of cognitive dissonance, then, in the decision to inundate the first two-thirds of the film with celebrity cameos. Everyone from Tony Bennett to Lady Gaga to Sean "Diddy" Combs to Usher shows up. They are almost universally distractions to what is actually not a half-bad plot, and, ironically, they are the type of people you think of when you think of people who are told what they want to hear all day by a sea of assistants and publicists.

While Burrell and Fey work just fine with their puppet cohort, Gervais does not. The caustic Brit seems disinterested most of the time, as if he knows his brand of sarcastic humor (which I actually enjoy in most settings) just isn't going to work here. Unlike his principal co-stars, and Segel and Adams before him, he can't seem to turn the trick of receding in to the background when it is time for Kermit, Gonzo, Miss Piggy and the rest to take center stage.

Aside from an unsettling fixation on showing the Muppets' lower halves that carries on throughout the film, Muppets Most Wanted does have its moments, especially during its final third. Everything -- from the humor to the musical numbers to the plot itself -- picks up considerably as Badguy and Constantine are exposed as frogs frauds by Walter, Animal and Fozzie Bear. (That's a pretty good Muppet trio to save the day, if you ask me.)

It's too bad that's buried behind a lethargic and largely aimless start that is more about injecting famous faces in to a shaky plot than about building steady momentum.