In 1941, on the island of Guernsey, four friends are stopped by soldiers for breaching curfew during German occupation. To avoid arrest, they say they were returning from a meeting of their book club, hastily named "The Guernsey Literary and Potato Peel Pie Society".
Five years later, in January 1946, the author Juliet Ashton is promoting her latest book, written under her pen name Izzy Bickerstaff. She has just been contracted through her publisher Sidney Stark to write stories for The Times Literary Supplement about the benefits of literature. Juliet receives a letter from Dawsey Adams, a Guernsey man who has come into possession of her copy of Charles Lamb's Essays of Elia and who wants to know where to find a bookshop in England to buy another book by the same author. He tells her that he is part of "The Guernsey Literary and Potato Peel Pie Society", which meets every Friday night. Juliet sends a book by Lamb and his sister, Tales from Shakespeare, in exchange for more information about the society and how it came into being.
Juliet decides she would like to write about the society and arranges to travel to the island, despite Sidney's reservations. Her American boyfriend Mark proposes before Juliet embarks on the ferry, and she accepts. Upon arrival at Guernsey, Juliet attends a meeting of the society where she is treated as a celebrity by the members: Dawsey Adams, Amelia Maugery, Isola Pribbey, Eben Ramsey, and Eben's young grandson, Eli. Juliet is told that Elizabeth, the founding member, is overseas. Her daughter Kit is being looked after by Dawsey, and calls him "dad". Juliet asks permission to write an article about the Society, but Amelia reacts negatively to the idea.
Instead of returning home as planned, Juliet remains in Guernsey to conduct research, telling the group that she is writing about the German occupation. Over the following days, she learns that Elizabeth had been arrested during the occupation and sent to Germany, but that her friends are still hoping she will return. Juliet asks Mark, who is in the armed forces, to try to locate Elizabeth. Juliet's landlady tells her that Elizabeth was no saint, hinting that she had been having sex with the occupying German forces in exchange for luxuries. Juliet asks Dawsey about the story, and he explains that he is not actually Kit's father. Her real father was Christian Hellmann, a German doctor who had worked with Elizabeth at the local hospital. Hellmann had been sent back to Germany, and died when his ship was sunk.
Mark arrives in Guernsey, and criticises Juliet for not wearing her engagement ring. He brings information about Elizabeth, and Juliet relays to the society the news that Elizabeth had been sent to the Ravensbrück concentration camp. There, she was shot and killed trying to protect a fellow prisoner. Juliet and Mark return to London but Juliet is unable to settle back into her previous life. She breaks up with Mark and starts to write about the society. When her manuscript is finished, she gives a copy to Sidney and posts another to the society. Dawsey reads her covering letter out loud to the group and realizes that Juliet has broken up with Mark. He decides to go to her, and departs for London. At the same time, Juliet arranges to return to Guernsey. She is just embarking on the ferry when she notices Dawsey on the wharf, and the two reunite. Dawsey is about to ask Juliet to marry him when she interrupts to ask him the same thing. He accepts.
Some time later, Dawsey reads to Kit from Tales from Shakespeare with Juliet next to him, both Dawsey and Juliet wearing wedding rings. As the credits roll, the Society holds another meeting off-screen, including Sidney, exchanging excerpts from books and discussion.