An unbiased summary:
Kermit lives in a swamp with his best friends, Croaker and Goggles. There's also a big bully bullfrog, I think his name is Blotch. Anyhoo, Goggles is taken by a pet owner, and an evil scientist/teacher is looking for frogs for his evil plans (aka dissection)! So Kermit and Croaker hit the road trying to save Goggles.
A biased summary: I thought this movie was really cute, and I fell in love with Goggles. However I found that the movie doesn't always flow quite evenly. And I didn't like the female dog character. Overall, you can tell that it is geared towards a younger audience, but I think it's cute, and even if you don't like it, it's sort of your duty as a Muppet fan to at least watch it.
Edit: fixed my poor grammar!!