Due for release in March 2014, EW describes the plot like this:
Director James Bobin returns with Ricky Gervais, Tina Fey and Ty Burrell anchoring the human cast, though numerous cameos will surely be announced before release as well.The Muppets...Again! ... finds the Muppet repertory company enjoying a triumphant world tour, only to become tangled up with a criminal mastermind named Constantine, who is out to steal an enormous diamond--and who happens to be a dead ringer for Kermit.