Catherine, an artistic twelve-year-old girl, has spent much of her life looking after her autistic eight-year-old brother, David, and teaching him to memorize rules for life. When Kristi, a girl her age, moves in next door, Catherine is worried that David's weird behavior will put her off. But not only does she befriend Kristi, she also befriends Jason Morehouse, a slightly older boy who goes to the same occupational therapy center as David.
Rules contains examples of:
- Ambiguous Disorder: Jason is mute, uses a wheelchair, and communicates via picture cards, but Catherine never learns exactly what his disability is and neither does the reader.
- Big Sister Instinct: Catherine once got in trouble with the bus driver for shoving David's bully, Ryan Deschaine.
- Character Tics: David flicks his fingers like he's playing a piano. He also shakes his arms when he's excited and flaps his hands when he's upset.
- Door Slam of Rage: Catherine slams the door to her mother's office out of frustration with being made to babysit David when she'd rather hang out with Kristi.
- Hates Being Nicknamed: Catherine keeps telling her dad not to call her "Cath," which reminds her of a baby cow.
- High-School Dance: Catherine attends a dance with Jason, where she overcomes her embarrassment over dancing in public.
- Innocent Inaccurate: David thinks Ryan is his friend. He doesn't understand that he's laughing at and not with him.
- Sensory Overload: David hears everything much louder than most people do. He covers his ears at any sudden noise.
- Speaks in Shout-Outs: David finds borrowing other people's words easier than providing his own. He especially likes Frog and Toad, and can hold entire conversations in Frog and Toad quotes.
- Teen Idol: Kristi has a crush on an idol named Jake, who appears in Teen People magazine.