When Bart became jealous of the attention Lisa was getting after she got a new hat, which she affectionately named "Sunny", Bart threw the hat out of the car window while Lisa was sleeping and into a scrapyard. That night, Bart's guilt appeared to him. Bart was adamant that he didn't feel guilty about throwing away Lisa's hat, and Bart's denial also showed up to protect him, but Bart's guilt snapped his denial's neck. Later, Bart woke up in the night to find that his guilt was still there. Bart told his guilt that he looked worse, and Bart's guilt told him that Bart's lack of remorse made him grow. Bart's guilt then got bigger and greener, scaring Bart. However, Bart still tried to take it in stride, telling his guilt that he liked gross things when his guilt ate his pillow and spat it out covered in green goop, and enjoyed being eaten and pooped out by his guilt too.
The next morning, Bart's guilt had grown even bigger. Bart lied to his family, telling them that he had slept well. Marge then asked if he had seen Lisa's hat, and Bart's guilt grew again. Bart told Marge that he didn't remember Lisa having a hat, causing his guilt to grow even more. Bart's guilt then blew his nose on the kitchen curtains and cooked his hand in the waffle maker...
When Bart betrays Lisa by throwing away a hat she becomes attached to, he has to deal with his guilt—literally, as it manifests to Bart. Meanwhile, Homer is revealed to be a chess savant.
