Lucy is the narrator and central protagonist of the story. A bolt of lightning strikes Lucy at age eight and turns her into a math genius—an acquired savant, one who becomes brilliant after experiencing head trauma—and by age 12, homeschooled by her grandmother, she’s finished her high school degree. Nana wants her to learn how to relate to others, so she sends her granddaughter to middle school for a year. Lucy tries to downplay her genius at school but can’t hide her obsessive compulsions about cleaning things before she touches them, tapping her foot three times, or sitting down three times in a row to stop her mind from obsessively reciting hundreds of digits from the number pi.

Lucy’s attempt to make friends at school drives the plot. Her adventures and misadventures with Windy, Levi, and the dog Cutie Pi force her to question her unrelenting OCD, help her to grow beyond a life of pure mathematics, and bring warm friendships into her life.

Large for her age, smart, a bit pushy, and cheerful, Windy befriends Lucy as a fellow eccentric. She wants to be an environmental lawyer, cares about animals, loves candy, and adores musicals.