Summer is supposed to be all about fun, friends, and baby-sitting, but Claudia Kishis life has just been turned completely upside down. When her beloved grandmother, Mimi, suffers a sudden stroke, everything changes for the entire family. Instead of spending her days running the Baby-Sitters Club, Claudia finds herself spending all her time Mimi-sitting and helping her grandmother recover. Claudia is trying incredibly hard to be helpful and kind, but she could really use some support from her older sister, Janine. Unfortunately, Janine is being completely impossible. She is entirely focused on her own smart-girl summer classes and cannot be bothered to help out around the house. Claudia feels like she is doing all the heavy lifting, and Janine is just being plain mean. As the summer heats up, tensions in the Kishi house reach a boiling point. Claudia must figure out how to care for the grandmother she loves while navigating the frustrating reality of living with a sister who simply does not understand her.