Marge and Lisa become addicted to true crime podcasts, which are gaining popularity. Homer Homer and Bart decide to visit Grandpa at the Springfield Retirement Castle and find that he has a new girlfriend named Vivienne St. Charmaine, a former TV star.
Later, during a romantic getaway cruise, Grandpa’s girlfriend Vivienne is reported dead from a fall aboard the cruise ship.
To return to its former popularity, Kent Brockman decides to create a podcast called "Guilty Grampa" that convinces Springfield, the family, and even Grampa himself that he is guilty of the crime.
Kent discovers that Vivienne had left all her money to Grampa in her life insurance policy. Grampa confesses and gets locked away in jail. Later Dr. Hibbert reveals to the Simpsons that Vivienne is alive: he had been tracking both hers and Grampa's movements via GPS chips he had implanted during their colonoscopy, and she had faked her death and fled to a Mexican resort. Hibbert and the Simpsons then confront Kent with the information ahead of the "Guilty Grampa" live finale, and successfully pressure him to reveal the truth. Though Grampa is released from prison, he is upset as he believed that Vivienne was the last love of his life.
In the final scene, Grampa encounters Vivienne, who reminds them that he was supposed to meet her in Mexico with the insurance money as they had earlier planned. The pair decide to hide out unnoticed together in the Springfield Retirement Castle until the publicity of the case dies down.