Homer, Lisa, Bart and Maggie are cleaning the garden, when Marge orders to them to go inside. After finally getting Homer indoors, the family sits and watch Kent Brockman's "Oops Patrol", where it is revealed Marge has discovered a funny headline and she is rewarded with a "Oops Patrol" shirt. Homer then gets jealous of the shirt and strives to win one for himself.
Homer spends the next evening searching for funny headlines in his bedroom. He then discovers an interesting article called: "The World's Biggest Pizza". The first lines of the article spells Homer's name out, which Homer gets notices, so he wakes up Marge and tells her that someone is trying to communicate with him. He discovers it is today's newspaper and he wakes Bart so they can go together.
Upon arriving at Four Street Overpass, the mystery person arrives, scaring Bart and Homer. She then reveals herself to be Mona Simpson and puts a "World's Best Grandson" sweater on Bart. At the Overpass Diner, she apologizes for being mysterious, but she had to be cautious due to the fact the government is still after for her for destroying Mr. Burns' germ lab. She used her links with the Springfield Shopper to communicate with Homer, using the story of the giant pizza to lure him. Bart then questions why Mona returned and she reveals it was because of the macaroni pencil pot Homer had made when he was five.
Chief Wiggum, Lou and Eddie arrive at the diner. Mona is recognized by Eddie and she, Homer and Bart flee the diner. Whilst in the car, Homer vows not to let the cops capture Mona again, but he crashes into the police station, and Mona is arrested. During Mona's court case, Mr. Burns tries to get Mona put on trial, but she is released, much to Homer's delight.
Mona then catches up on Homer's childhood; bathing him, watching him in the school play, knitting for him, teaching him to ride a bike and watching a reenactment of the birth of Bart. In Moe's Tavern, Mona catches up with Lenny and Carl. Mr. Burns then renames his Germ Lab to "Grandma Simpson Peace Museum and Kid-teractive Learnatorium". He then asks Mona to be the first to sign the visitor book, but as she signs it, she mentioned of using used fake names in the visitor books of theme parks, which is a federal offense. The FBI then arrests Mona, which deeply saddens Homer.
Homer reminisces his time with Mona and plans to break her out of prison. The following day, Bart and Homer break onto the bus that is transferring the prisoners to jail. Homer then hijacks the bus and all of the prisoners (except Mona) get off. The police see this and give chase to the bus, but Mona pushes Homer out of the bus onto a bed, which rolls into some large cacti. The bus then nears a cliff with the police in hot pursuit, when Homer watches in despair as the bus flies of the cliff and lands in a large lake, which is then covered by an avalanche. At Mona's funeral, the coffin is lost and Homer kicks over Frank Grimes' grave.
Homer chooses to see if his mother has communicated with him again. Eventually, he finds an article which reads: "IMOK". He then goes to sleep and it is revealed there is an article about a giant taco, which reveals that Mona escaped from the bus and is fine.