Another new and ordinary day dawns in Springfield. Another new episode of Itchy and Scratchy is broadcast on tv, another prank was executed by Bart, another "haa haa!" yell heard from Nelson and another (try a dozen) humiliation of Homer Simpson seen.
Lisa Simpson is concerned about the rapid polluting in the town, as always, and as usual, no one really listens to her. That is until she meets her dream boy, who is as conserned about nature as she is. Convinced that she'll get her message through, she arranges a seminar to save the Sprigfield lake that's been used as a general waste dumping site and is threatening the well-being of the entire town. Indeed, the mayor orders the lake to be fenced and no one to pollute it any longer.
In the meanwhile, in some moment of madness, or just on a whim like usual, Homer has gotten himself a pet pig. He just adores that little sucker, and treats it like the son he never had - or so he thinks as he's clearly forgotten Bart's existence again. Bart's sad that his own dad wont acknowledge him, talk to him or choke him when he's angry anymore, so he seeks a father-like figure elsewhere, and finds one in diddly-diddly Flanders.
Homer, for the sake of stopping Marge whining, has built a pig crap tank. But since his pet craps a lot, although he helped a little himself, the tank fills up fast, and can't be kept in their yard. Homer sets out to dispose of the tank, but before he can't even think.... of anything, he gets a call that there's a free donut-day in a nearby donut shop, and they'll run out fast! D-OH! So, he decides to just dump the crap tank in the lake nearby, nobody's going to notice it anyway, right?
Here's where the trouble starts. The lake becomes so polluted that new lifeforms are born out of it. The Enviromental Protection Agency decides to isolate the whole town of Springfield, putting a huge dome over it, with all the residents trapped inside! The stores start to run out of food and supplies, and the residents want out. The Simpsons family face a scary threat when the other residents find out that it was Homer who spoiled the lake and the town's ordinary life. Being chased, the Simpons family find a secret passage where they rumble outside, to the other side of the dome and to freedom. Now all they have to do is hide, and what a better place to do that than - Alaska?
Homer gets the cold shoulder from Marge and the kids for ruining their lives, yet again. Marge wont put up with him or Alaska for long, and insists that they save Springfield, in a way or another. Homer disagrees and finds the local bar, while Marge leaves with the kids to Springfield. On sobering up and watching a touching video from Marge, Homer gets up and sets for Springfield, by any means necessary. He rides a dog sled, then passes out after getting lost, wakes up in an native american tent and sees visions, but ultimately gets back to Springfield and finds his family.
The goverment has decided to bomb down the whole town inside the dome, along with it's residents, to prevent a natural disaster. There's only one man who can save the town as did he ruin it in the beginning, and that is Homer Simpson! He will use super glue, mad bike skills and his regained father-son relationship with Bart to get rid of the bomb and save Springfield so that he can doom it to destruction yet again one day!
Do remember to sit through the ending credits, as there's still some fun to come until the very end!