Set in Cape Elizabeth, Maine, "One Summer" depicts two love stories—one lost in time and the other blossoming in the present.

McKenzie (Mack) Harper rents a charming summer cottage, hoping for a relaxing vacation. While there, she stumbles upon a 1947 diary that draws her into a world of love and heartbreak.

Each entry is written by Rose Degan, a nineteen-year-old who falls in love with a man her parents disapprove of—Cape Elizabeth firefighter Jacob Evans, whom she meets on the cliffs while watching the rescue of a collier's crew after a violent storm.

Jacob's grandson, Dean Evans, intercepts Mack's note to his grandfather and becomes intrigued by the mysterious woman and her diary. Intent on avoiding his mother's matchmaking—her only goal is a grandson—he heads to Cape Elizabeth, unaware that his life is about to change.

All of these characters come together in a story of love and friendship.