Pertice McIlveen, a young Ontario woman who loves Hemingway and hates hats, receives a mysterious key in the mail. Accompanied by her best friend Es, she travels to Gannet Island off the coast of New Brunswick to find the door it fits into. There she discovers a charming cottage by the sea has been willed to her by a secret benefactor identified only as PM, on the condition she wears the hats that come with it. She accepts the challenge, leaving behind her life in Toronto and moving into Honeysuckle Cottage. As she seeks to solve the mystery of PM, Pertice is gradually changed by the hats she wears and the islanders she meets. With the help of Charlotte, the proprietor of the local bed and breakfast, her artist husband Will, and two men caught between land and sea, Pertice discovers a new kind of grace under pressure.