Arthur thinks that babysitting is an easy job until D.W. volunteers him to babysit Tommy and Timmy Tibble, twins with a bad reputation of being destructive and mean. After some initial struggles, Arthur gets the twins to behave by telling them a scary story. Arthur's parents are hosting their annual family reunion. Arthur tries to avoid his cousin Mo, who has bullied him during past reunions, while his parents deal with their eccentric relatives. When a thunderstorm forces the Reads to spend the rest of the party indoors, Arthur learns that Mo has been looking for him to ask if he could give her a piano lesson.