Polly is introduced in The Magician's Nephew—which is the sixth book in the series to be published, but is the first in the internal chronology of Narnia.
Polly uses the attic as a hide-out where she drinks ginger beer and is gradually writing a private story that she does not share with Digory.
One day, while exploring the inner roof space in the row of terrace houses, the children accidentally enter the study of Digory's Uncle Andrew.
He is initially an alarming person in his own domain, but he successfully ingratiates himself with Polly and offers her a yellow ring as a gift which causes her to vanish as soon as she touches it.
Uncle Andrew believes Polly has now been sent to another world and blackmails Digory into following her with another ring in order to bring her back.
After working this out, Polly and Digory decided to try a nearby pool and travel to the ruined city of Charn on a dead world that has a dying sun.
Polly's stolidity, friendship, morality, and sense of caution counterbalance Digory's drive, aggression, and grief.