What is the plot of the Simpsons Movie?

I've seen all the trailers and seen pictures of toys based on the movie. In the trailers, a mob is chasing Homer and in an animatic Homer dumps a silo full of pig poop into Lake Springfield, so I can guess those two events are connected. There is also a toy which shows Bart and Ned Flanders seeing a mutated animal behind a bush at Lake Springfield which leads to Homer possibly contaminating the lake with pig poop. Also, in the trailers, for a split second you can see a huge gl dome covering Springfield, which is being done because the town is polluted and hazardous? I'm just guessing off of what I see here. Do any of you know the real plot of the film?