Dewey Kerrigan, who is 10 when the novel opens and 12 when it closes, is an introverted child who loves science, mechanics, and math. She is characterized as an unusual young girl (for the 1940s context) in her love of reading The Boy Mechanic and building radios and gadgets. Since the beginning of the war in 1939 (when Dewey was seven), Dewey has moved around a lot because of her father’s work as a scientist, which makes him unavailable to act as a stable primary caregiver. She lives first with her Nana and then with her Nana’s unfriendly neighbor, Mrs. Kovack, before being relocated to Los Alamos to live with her father in 1943, much to Dewey’s delight. Dewey’s intellectual pursuits are a constant that provide stability in her life and help her to feel connected to her father.

Dewey is ridiculed and socially ostracized at the school on the Hill, where she is known as “Screwy Dewey” due to her habit of sitting alone to build mechanized gadgets created from discarded scraps from the dump. One of her bullies in the schoolyard is Suze Gordon. Much to Dewey’s dismay, she is sent to live with the Gordons when her father leaves on an extended work trip.