Returning from boarding school to her home in Friendville, sixteen-year-old Mary Rose is looking forward to a peaceful, relaxing summer. Her grandmother in Colorado falls ill, however, and Mary Rose's mother has to go care for her, leaving Mary Rose in charge of her two younger siblings. Just before she leaves, Mother receives a letter from her brother, who informs her that his work is sending him to Japan, and he is entrusting his motherless children to her. Mary Rose's four cousins are already en route to Friendville, and it is up to Mary Rose to take on the responsibility of caring for her "ready-made family" for the summer. Between parties and fun, illnesses, injuries, and a nearly-disastrous beach trip, Mary Rose experiences both the responsibilities and the joys of "growing up."