Heidi is 12 when the novel opens. She lives in an apartment in Reno, Nevada she shares with her mother. Heidi does not know any family members besides Mama. Bernadette, next-door neighbor whose apartment connects to Mama’s and Heidi’s, serves in a caretaking role to them both. Heidi does not know anything about her mother’s life or history before Bernadette discovered Mama in the hall near her door and infant Heidi crying in Mama’s arms. Heidi does not know her own birthday or how she and Mama came to live in an apartment where rent and utility bills never arrive. Her exposure to the outside world is limited from the time she is a small child due to Mama’s condition and Bernadette’s agoraphobia. Bernie instructs Heidi in reading at home; Heidi does not attend school. Errands consist of the market and stores on the same city block until Heidi is old enough to cross the road. Heidi would like to venture further away from the apartment building, but Mama cannot board the city bus; the one time Heidi tries to persuade her to do so, Mama has a strange panic attack and loses control.

Now that she is 12, Heidi cannot help but question their situation and wonder how it came to be that the two of them live in Reno rent-free.