Homer decides to end it all. He kisses Marge goodbye for work that morning, but then instead of going to the Nuclear Plant, he stops outside of Springfield Elementary and gets out of the car. He pops the trunk and pulls out a trenchcoat, an AR-15, and a sawed-off shotgun, and starts to walk towards the school when he spots Sideshow Bob doing the same. Sideshow Bob is finally resolved to kill Bart, and he and Homer join forces and storm the school. Lockdown alarms are going off as Homer and Bob execute Bart, Lisa, and the others. Then, as the police arrive outside, Homer and Bob decide to go out in a blaze of glory by charging back out the front door at the cops. Homer ducks back in at the last second so that he won't be seen, leaving Sideshow Bob to charge out alone, getting gunned down by police as he realizes that Homer betrayed him.
Homer then drives home just in time to see Marge watching the news report on TV. He beats her with a hammer, but doesn't kill her, because he wants her to experience pure agony while she bleeds out. Then he drives to the Nuclear Plant to continue his shooting spree. Marge calls the cops, but it's too late, and by the time they realize that there's a second shooter, Homer has already reached the reactor. The last that anyone sees of Springfield is a bright flash and a mushroom cloud, and then the screen goes black.
It's a two-partner finale, and the second part is just thirty minutes of everyone in Springfield staring at the camera with realistic bloody eyes while the main theme plays in reverse. During the end credits, Matt Groening himself walks into the screen to confirm that Dead Bart was real.
Then the new Family Guy episode starts with Brian and Peter watching the same episode on TV.