The protagonist and the narrator of Rules is 12-year-old Catherine, who begins the story at the beginning of her summer break. She lives with her family on the picturesque Maine seacoast in a small town. Catherine finds summer to be a dreary time. Her best friend, Melissa, has departed for her regular three-month visit with her father in California. Since Catherine’s mother works part-time from home and her father is a more-than-full-time pharmacist, the family has no vacation plans. Instead, she’ll be spending her time babysitting her younger brother, David, who regularly attends occupational therapy for his autism.

Though the summer seems to hold little promise for Catherine, she has high hopes for a newly arrived friend. Mrs. Peterson is moving in next door, and she has a daughter named Kristi who is Catherine’s age. Catherine makes a mental list of the fun things to tween girls can do: swimming in the nearby pond, riding bikes, working on art projects, and working out Morse Code on flashlight from house to house at night.

Catherine is also intrigued by a boy name Jason Morehouse who is a year or two older than she is. She sees him in the waiting room of the occupational therapy clinic when she accompanies David for his regular appointments.