In January 1946, 32-year-old Juliet Ashton embarks on a cross-country tour across England to promote her latest book. Written under her pen-name Izzy Bickerstaff, the book is a compilation of comedic columns she wrote about life during World War II. Despite the fact that she initially wrote under the name Izzy Bickerstaff during the war, Juliet writes to her publisher that she wants to retire the character and write under her real name.
On her tour, Juliet is greeted with flowers from the mysterious Markham V. Reynolds, Jr. Her best friend and publisher, Sidney, warns Juliet that Mark is a wealthy American trying to establish a publishing empire and looking to poach her. Reynolds makes it clear that he is a fan, and she and Reynolds soon begin dating.
Juliet receives a letter from Dawsey Adams, a complete stranger from Guernsey who has come into possession of her copy of Essays of Elia and who wants to know more about the author, Charles Lamb. Juliet helps to send him further books by Lamb. She is also intrigued that Adams is part of The Guernsey Literary and Potato Peel Pie Society and inquires about the group's name.
After learning that the society began as a cover for residents breaking curfew during the German occupation of Guernsey, Juliet begins a correspondence with several members of the Society, hoping to work them into an article she is writing on the benefits of literature for The Times Literary Supplement. Juliet also learns that Elizabeth McKenna, the Society's beloved founder, was arrested and sent to a prison in France by the Germans and has yet to return home. The members of the Society are raising her child, Kit, among themselves until Elizabeth returns.
As she continues to write to the members of the Society and they to her, Juliet begins to plan a trip to Guernsey to conduct research for a book about the group and their experiences of the war. Mark proposes as Juliet is preparing to leave for Guernsey and she delays giving an answer, not wanting to repeat the error of her previous engagement.
In Guernsey, Juliet is treated like an old friend and soon helps to watch Kit. She is also there when the members of the Society receive a letter from Remy Giraud, a French woman who was in the Ravensbrück concentration camp with Elizabeth. She informs them that Elizabeth is dead, but several members go to see her and encourage her to visit Guernsey with them, to which she eventually agrees.
Juliet decides to center her book on Elizabeth's experiences on Guernsey during the occupation, as told by her friends. While she is writing, Juliet is visited by Mark. Realizing that she has feelings for Dawsey and has since they first met, Juliet definitively rejects Mark's second proposal.
As she continues to write, Juliet also realizes that her time spent with Kit means that she now thinks of Kit as a daughter and wants to adopt her. She also longs to be with Dawsey but fears that he has fallen in love with Remy.
Remy eventually announces her plans to return to France and train as a baker in Paris. Isola Pribby, a member of the Society, believes that Dawsey is in love with Remy and, using Miss Marple as a model, offers to clean Dawsey's home to find proof he is in love with Remy to convince her to stay in Guernsey. Isola's plan is a failure, and she goes to Juliet to complain that she was unable to find anything that would signify his love for Remy, but instead found numerous pictures and tokens that belong to Juliet. Realizing that he is pining for her, Juliet runs to Dawsey and asks him to marry her.
Juliet ends by asking her publisher and friend Sidney to return to Guernsey in time for her wedding in a week's time.
