The narrator starts by telling us that she and her family didn't always live on Mango Street – they've moved around a lot.
The family's new house on Mango Street is all theirs – previously they'd been renting apartments.
The narrator says her family had to leave their last apartment in a hurry because the water pipe broke and the landlord wouldn't fix it.
Mama and Papa had always promised their children that someday they'd live in a real house with working appliances and a fancy staircase.
The house on Mango Street is a disappointment – it's not big and fancy at all, and all six family members have to share a bedroom.
The narrator tells the story of the moment she realized she had to have a real house: one day, while she is playing in front of the apartment on Loomis, a nun from her school passes and asks where she lives. She points to the third floor of the worn, paint-peeled building, and the nun says: "You live there?" The narrator decides she needs to have a real house that she can point to without feeling ashamed.
The house on Mango Street is not that house.
The narrator's Mama and Papa say the house on Mango Street is temporary, but the narrator is dubious – she "[knows] how those things go" (1.11).