Juliet, a young author living in London, writes to Sidney, her friend and editor: she's lost interest in her current book project and no longer wants to write under the pseudonym Izzy Bickerstaff, which she used during World War Two. Soon after, Juliet embarks on a tour to promote the book she wrote with her pseudonym, Izzy Bickerstaff Goes to War. While on the tour, she has a brief run-in with Gilly Gilbert, a journalist who publishes only gossip. Gilly tries to get Juliet to speak ill of her engagement with Lieutenant Rob Dartry, but Juliet throws a teapot at him. Later, Juliet confides in Sidney that she broke off her engagement with Rob because he tried to move her books to the basement and replace them with sporting trophies. While on the tour, Juliet begins receiving flowers from a man named Mark Reynolds. Sidney calls Juliet home from the tour early, as the Times wants her to write three articles about reading.
A man named Dawsey Adams starts writing to Juliet. He has one of her old copies of a book by Charles Lamb and wants help finding others by Lamb. He lives in Guernsey. He piques her interest by mentioning that he's part of a literary group called the Guernsey Literary and Potato Peel Pie Society, which began with an illegal roast pig during the German occupation. Juliet, intrigued, helps Dawsey find more books by Lamb and asks him to tell her about the Society and the pig. He obliges and explains that the Society began after the pig roast when he, John Booker, and Elizabeth McKenna were caught out after curfew. Elizabeth made up the Society as a cover story and they had to actually start it, since the Germans wanted to attend. Juliet asks if she can write about the Society for her Times articles, so Dawsey asks his friend Amelia Maugery to write to Juliet.
Amelia is concerned, after reading Izzy, that Juliet will make fun of the Society, but Juliet asks Reverend Simon Simpless and Lady Bella Taunton to provide character references. Satisfied, Amelia tells Julia more about the Society and her friend Isola also writes to Juliet. Juliet attempts to write to Sidney about her new pen pals, but discovers that he ran off to Australia to care for their friend Piers, who mysteriously showed up in Australia after being shot down over Burma in 1943. In Sidney's absence—which becomes even longer when Sidney breaks his leg—Juliet begins dating Mark. Mark is handsome, rich, and American, though he's a bully.
Juliet receives letters from several other members of the Society. Eben tells Juliet about how he wasn't enthusiastic about reading until he discovered Shakespeare. He also tells Juliet about Eli, his grandson, and how all the children of Guernsey were evacuated before the Germans invaded. Clovis writes that after discovering poetry, he was able to court and marry his wife, the Widow Hubert. Booker reads only The Letters of Seneca and says the Society saved him from the life of a drunk.
Through these letters, Juliet begins to learn about Elizabeth McKenna. She first came to Guernsey as a girl, as her mother worked for Sir Ambrose. Elizabeth ran wild and befriended Eben's daughter, Jane. She returned to the island right before the invasion to close up Sir Ambrose's house and chose to stay, as Jane was pregnant and on bed rest. Jane died when the Germans bombed Guernsey. Elizabeth became a nurse, concocted a plan for the partially Jewish valet Booker to impersonate his employer, Lord Tobias, and fell in love with a German soldier, Christian Hellman. Everyone speaks well of Christian. He helped the islanders and planned to return after the war to marry Elizabeth. However, a few months before Elizabeth gave birth to their child, Kit, Christian was sent to work in a hospital in France and died. In 1944, Elizabeth was imprisoned and sent to the mainland for helping one of the Todt workers. Nobody knows where she is now. The Society shares the duties of caring for Kit, who is now four years old. Juliet also receives letters from a Miss Adelaide Addison, who believes it's her duty to make sure Juliet knows that Elizabeth was a self-important upstart.
As Juliet corresponds with her friends in Guernsey, she also continues to date Mark. He takes her out every night and won't accept her refusals if she wants to stay in. Susan and Juliet beg for Sidney to come home, and Juliet tells Sidney of her research into Guernsey. Dawsey recounts to Juliet how his friendship with Christian began, and the two talk about their love of reading and Charles Lamb. Amelia writes Juliet about the Todt workers: the slave laborers who built the bunkers on the island. The Germans starved them, and the bunkers remind Amelia of the atrocities the Nazis committed.
Isola asks Juliet to answer some personal questions. Juliet answers all of them and says that she was orphaned at age twelve and went to live with her great-uncle, Dr. Ashton. She tried to run away twice, after which he sent her to boarding school where she met her best friend, Sophie. Juliet and Sophie lived together after school and worked in bookstores, and now Sophie lives in Scotland with her husband and son. Juliet is her son's godmother. Juliet says that while she has a suitor, she's not sure she likes him.
Mark insists on taking Juliet out and proposes. She insists in a letter that she didn't actually refuse; she just wants to think. Later, Juliet writes and tells Sophie about the proposal. Mark made Juliet cry by berating her to say yes. She wonders if she's stupid for not accepting and says she wants to go to Guernsey to write about the occupation. She asks Sidney about going to Guernsey and he gives his permission. Everyone in Guernsey is thrilled to have Juliet and they set about preparing Elizabeth's cottage for her. Isola tells everyone she knows to write Juliet about the occupation and the letters flood in. An Animal Lover writes about how the U.S. euthanized most of Guernsey's house pets, while Sally Ann Frobisher writes about Elizabeth's skill at nursing. Micah Daniels tells Juliet about the boxes of food that the British sent and that the Germans, surprisingly, didn't steal. In preparation for Juliet's arrival, Booker writes to share that he actually spent a year in a concentration camp after his ruse was discovered. Mark tries one last time to stop Juliet from going to Guernsey, but she goes anyway.
Juliet recognizes everyone immediately upon her arrival. Though Kit is stern at first, she warms to Juliet by the end of the evening. Juliet tours the island and attends a meeting of the Society. She receives more letters from locals about the invasion and when she hears that Sidney is in London, she invites him to visit. She confides in him that Kit has been staying with her and they play a morbid game called Dead Bride. She asks Sidney to send paper dolls for Kit. Sidney's new secretary, Billee Bee Jones, sends the dolls.
The Society receives a letter from a Frenchwoman named Remy. Remy writes to say that Elizabeth became her friend at the Ravensbrück concentration camp and was executed in March of 1945. Elizabeth stood up for a woman who was beaten for menstruating. Remy's nurse, Sister Touvier, includes a note saying that Remy is very ill, but she hopes the letter will help her heal. Amelia and Dawsey ask if they can visit, and Juliet's friends on Guernsey mourn. Juliet worries for Kit's future, as she's now an orphan. She also forbids Mark from visiting.
Dawsey and Amelia visit Remy and Juliet cares for Kit. Remy is exceedingly frail, but she wants to move to Paris. Dawsey, however, wants Remy to come to Guernsey. Juliet tries to keep things normal for Kit and asks Sidney to visit. He agrees to come over the weekend. While Sidney is in Guernsey, he writes Sophie that Dawsey and Juliet are clearly in love with each other, and Kit loves Juliet. After returning to London, Sidney reads Juliet's manuscript and suggests she tell Elizabeth's story. Juliet sets to work asking everyone about Elizabeth and discovers some of Elizabeth's drawings.
Juliet writes to Sophie and admits that she loves Dawsey—but Mark showed up unannounced and ruined everything. In another letter the next day, Juliet says that she turned Mark down for the last time when he said that the locals are forcing Juliet to care for Kit. Soon after, Juliet discovers that Elizabeth was arrested with a man named Peter Sawyer for helping a Todt worker named Lud Jaruzki. Peter was allowed to return to Guernsey, but he never heard from Elizabeth or Lud after their arrest.
Dawsey convinces Remy to come to Guernsey and asks Juliet if she'll spend time with Remy, as her sunny disposition will be good for Remy. Juliet takes this as proof that Dawsey doesn't love her. However, she does find that she likes Remy. During this time, Sidney sends Isola a book on phrenology. Not long after, Isola reads letters written to her Granny Pheen that Juliet believes were written by Oscar Wilde. An expert confirms this, and Isola asks Sidney if she could write the preface if he publishes the letters. Billee Bee tells Juliet that Sidney is sending her to fetch the letters, and Juliet confides in Sophie that she'd like to try to adopt Kit. A few days later, Susan tells Juliet to not let Billee Bee take the letters: she's in cahoots with Gilly Gilbert. Fortunately, Isola and Kit locked Billee Bee in Isola's smokehouse before she could steal the letters.
Juliet writes to Sidney and recounts a horrific event: while walking along the cliffs, Remy suffered a violent panic attack at the sight of a German Shepherd. Dawsey explained later that the guards used to set dogs on prisoners. Juliet begins to wonder if Dawsey loves her, and Sidney plans another visit to the island. Soon before Sidney's arrival, Amelia gives Juliet her blessing to adopt Kit and Kit shares a box of secret treasures with Juliet. It contains mementos from her parents. Juliet and Dawsey have dinner, but Juliet is distraught when she sees Dawsey with Remy the next day, purchasing a suitcase.
In a notebook given to her by Sidney, Isola states she's going to observe her friends, just like Miss Marple. She notices that something seems amiss with Remy and Dawsey, but can't figure out what. Isola observes that when Sidney arrives, he and Dawsey look at each other strangely. She's worried when, at Eben's party, Juliet falls back when Eben announces that Remy is returning to France. She sees Dawsey looking sad and decides he must be in love with Remy, but is too shy to tell her. Isola offers to clean Dawsey's house so she can look for clues of Dawsey's love. Isola finds no signs that Dawsey loves Remy, but she finds letters, photos, and a handkerchief of Juliet's. Distraught, Isola tells this to Juliet, who promptly asks Dawsey to marry her. Juliet writes Sidney a final letter asking him to return to Guernsey so he can give her away at her wedding.