Hat Girl is the story of Pertice McIlveen, a young Ontario woman who loves Hemingway and hates hats. She receives a mysterious key in the mail and, accompanied by her friend Es, travels to Gannet Island off the coast of New Brunswick to find the door it fits into. There she discovers Honeysuckle Cottage has been willed to her on the condition that she wears the hats it contains. Over the course of a year, she is changed by the hats she wears and the islanders she meets, including Charlotte Murdoch, the proprietor of the local bed and breakfast, her artist husband Will, and Clive Ferguson, who has been driven to the mainland by an abusive father but returns to help her overcome fear with faith. Each of the chapters is named for a different kind of hat and begins with an epigraph from the glossary of Ernest Hemingway's book on bullfighting, Death in the Afternoon, where Pertice begins her journey to a new kind of grace under pressure.