Let's face it. If you grew up when I did, you love the Muppets. Like most Muppet movies, there are songs, surprise guests spots, a bad guy, visual gags galore, and an extremely loose plot.
The plot is that Kermit is replaced by the evil Constantine, the world's most dangerous frog. Kermit ends up in a Russian gulag being run by Nadya played by Tina Fey. Meanwhile, the rest of the gang are easily fooled by the bad impression of Kermit by Constantine and taken on a world tour lead by Constantine's secret "Number 2" Dominic Badguy played by Ricky Gervais.
There is a great song and dance number from Gervais and another fun one from Tina Fey. Even Ty Burrell, who plays Interpol Inspector Jean Pierre Napoleon, gets a song, although I must admit, the whole Sam the Eagle and Jean Pierre riff on Inspector Clouseau was a fairly weak running gag.