Milhouse and Bart loosen every bolt and screw in Springfield Elementary, leading to mass chaos when the building and its contents fall apart. Milhouse is apprehended by Principal Skinner, is suspended from school for a week (and subsequently grounded). Bart, whose involvement with the prank was not discovered, promises to visit Milhouse every day. Homer drops Bart off at Springfield Retirement Castle to visit Grandpa. There, Bart is immediately smitten with a kind and charitable girl named Jenny. Bart makes a concerted effort to appear "good" to Jenny, demonstrating his newfound good nature by defending ducklings and eventually inviting Jenny over for dinner. However, Milhouse shows up on the Simpsons' doorstep and threatens to reveal Bart's true, dark nature because Bart forgot to visit him during his suspension. Milhouse begins appearing on Bart and Jenny's outings, each time hinting at Bart's misdeeds. Eventually, Bart confesses to Jenny that he was actually bad before he met her and only pretended to be good to start a relationship. He continues to say that he is changed completely because of being with her. Jenny, though momentarily pleased by Bart's honesty, realizes she cannot forgive or trust Bart, and angrily dumps him. At home, Bart cries, and Homer & Marge console and reassure him that girls come and go, but he still has his family, only for him to cry even harder, and for Homer to shed tears too.
Meanwhile, Lisa is assigned to write a report on what Springfield will look like in the year 2059. When she discovers online reports about soap for drinks instead of water, a world war over a tiny drop of oil, a parking lot yet to be filled forever and the last polar bear committing suicide by hanging himself, she is filled with anxiety and depression and terrifies her classmates with her dark visions of the oceans rising from global warming, turning humanity and the lowlands into a desert and darkness falling upon Nineveh. Homer and Marge take her to a psychiatrist, who prescribes Lisa "happy pills" known as "Ignorital". Lisa is initially skeptical, but after taking her first pill she loses touch with her problems and sees pollution (as well as everything else) as smiley faces. In her love-induced stupor, she nearly kisses a running fan held by Maggie, until Marge finally intervenes by deciding that she wants Lisa back to normal, and should not take the pills anymore. She tosses them in the wastebasket, where they are promptly eaten by Santa's Little Helper. When Maggie (most likely doing it intentionally) holds the fan up to him, he (off screen) licks the fan himself.
Heartbroken, Bart is at the Kwik-E-Mart, drowning his sorrows on a couple of Shrek Squishees. Soon, a back to normal Lisa tells Bart that they can't be in despair about certain issues and should just move on. Bart decides to take Lisa's advice, and leaves after buying a bouquet of roses, and apologizes to Jenny for lying to her about his true character and Milhouse for his neglect to visit him.
In the end, Bart gives the roses to Milhouse, and Jenny is never seen again (having rejected Bart's apology earlier off-screen). The two friends reconcile and play a prank together, repeatedly driving a Zamboni over the floor of the school until it is as slippery as ice. They then wait for the bell and watch as the school kids slide around while fake snow falls simulating an out-door ice rink.