Rules starts off with Catherine, a 12-year-old girl, trying to drag her brother, David, to the car. David has autism, and he has to go to occupational therapy, or OT. However, he won't leave the house. Catherine decides to take a detour and talk to the moving men beside her house. She wants to know when her new next door neighbor, also a 12-year-old girl, will move in. She finally drags David to the car and they go to OT.
When they get to OT, she starts to sketch Jason, a boy who can't talk. He tells his mother, and she yells at Catherine. However, when she gives him the picture, they become friends.
When she finally sees Kristi for the first time, David is screaming. Kristi's first words to Catherine are "Is he okay?" Later, when she wants to meet them for real, they are busy unpacking or not in the house.
The next time Catherine goes to OT, she agrees to help Jason by making more of his speech cards. While making the cards, David needs her to get a toy out of the fish tank, and she sees Kristi talking to Ryan, a very rude boy.
One day, Catherine's mom is making hamburger patties, as she intends to invite Kristi and her family over for a picnic. However, Kristi is at her dad's house and they fail to meet again.
Catherine goes to OT again, and she gives Jason her new cards. They talk, and Catherine agrees to make a lot more cards for him. While making the cards, Kristi is invited to her house by Catherine's mom, and they finally meet for the first time. They talk for a very long time and play basketball, which means that Catherine has to miss OT. Catherine's mom brings back a present for Catherine from Jason, which is carrots for her guinea pigs. The next time at OT, she brings the cards and a surprise for Jason, which is one of her own guinea pigs. While they go home, Catherine gets card stock and a paper cutter to make Jason more words.
When they get home, Kristi is in the front yard with Ryan, and Ryan teases David with an empty gum wrapper.
The next week at OT, Jason wants Catherine to push Jason around in his wheelchair as fast as she can go, because he wants to know how it feels like to run, and she does.
The next morning, Catherine and Kristi go to the pool and swim, until Ryan comes and messes up the whole morning by making Kristi go with him instead of Catherine. They ask Catherine to go to the community dance, but she declines the offer, saying that she doesn't dance unless she is alone or the room is pitch black.
At OT, Jason and Catherine go for a walk. When Catherine sees Kristi, she hides so Kristi doesn't see her with a handicapped boy. Jason invites Catherine to his birthday party, which is the same day as the dance. She accepts, thinking that is will be a good excuse to not go to the dance. Catherine helps Kristi make posters for the dance. Kristi is rude that Catherine doesn't want to go to the dance.
Catherine goes to buy Jason a guitar for his birthday, and Jason has a wheelchair that has a joystick. When she goes to the party, she lets slip that there is a dance that night. Jason asks her if she wants to go, but she hesitates. Jason figures that she just doesn't want to go with a handicapped boy, and he becomes very angry.
When Catherine finally convinces him to go to the dance, she sees Kristi. Kristi is very rude that Jason is handicapped, and that Catherine didn't tell her that he was handicapped. Catherine and Jason dance, and Catherine realizes that her true friend is Jason, not anyone else that doesn't accept her or her friends.
