The following version of this book was used to create the guide: Perry, Sarah. Enlightenment. Mariner Books, 2024.

Sarah Perry's novel Enlightenment is set in the small town of Aldleigh, located in Essex, England. The novel straddles multiple temporal eras and is written from the third-person point of view. For the sake of clarity, the following guide relies upon the present tense and a streamlined mode of explanation.

In 1997, Thomas Hart is 50 years old. He has lived in Aldleigh since he was born and writes for the local Essex Chronicle newspaper. He is also a member of the Bethesda Chapel community and spends his time with his fellow congregants and his close friend, 17-year-old Grace Macauley. When Grace was born, Thomas immediately fell in love with her. He had wanted to leave Bethesda, fearing the church community would reject him if they discovered that he was gay. However, he decided to stay in the church because he wanted to care for Grace.

Grace is the daughter of Ronald Macauley, the Bethesda minister. Her mother Rachel Macauley died in childbirth. Anne Macauley, her aunt, has acted as her mother figure. Grace does not have any friends other than Thomas, and she spends most of her time between her home and Bethesda. She dresses in vintage petticoats, rarely washes her hair, and speaks in formal, antiquated English. Grace does not question her world until she encounters a young boy named Nathan one Sunday.

Grace and Thomas are sitting in the Bethesda pews together when a golf ball flies through the window, smashes the glass, and hits Grace. Grace is disgusted by Nathan's interruption, but she is also intrigued by the handsome young man. Over the following weeks, she and Nathan start to spend time together. They take walks, smoke cigarettes, and talk about their lives.

Meanwhile, Thomas tries to dismiss his anxieties over Grace's relationship with Nathan so that he can focus on his most recent writing and research projects. James Bower, a curator from the Essex Museum, contacts Thomas about the ghost at the local Lowlands House. With the museum, James is renovating the old house and wants Thomas's help solving the mystery of its last inhabitant, Maria Vaduva. The men start to spend time together, pouring over Maria's diaries and letters in an attempt to understand her life and death. They become particularly excited by her story when they discover that she was an amateur astronomer who made important scientific discoveries.

One night, Lowlands House catches on fire. The town races out to see what is happening. James and Nathan end up running into the burning house, convinced that the woman who has been squatting there is in danger. Grace falls to her knees and begs God to save Nathan, promising that she will get baptized if he survives. Nathan does survive, and Grace goes through with the baptism. However, during the ceremony, Thomas confronts Nathan and tells him to leave Grace alone. Nathan disappears from her life thereafter.

In 2008, Grace returns to Aldleigh for the first time in years. She left Bethesda and has been living in London. She comes back to see Thomas and say goodbye to Anne, who is dying. Five days after Anne's death, Grace and Thomas take a walk, and Thomas admits that he sent Nathan away years prior. Furious, Grace cuts Thomas out and slanders his name among the Bethesda congregants.

In 2017, Grace is living back in Aldleigh and working as a seamstress. Meanwhile, both she and Thomas are nursing broken hearts. Thomas thought he and James might have a future together years prior, but he discovers that James has never had any intention of leaving his wife to be with him. Then one day, Grace runs into Nathan in town. They start up a texting conversation, and Grace discovers that Nathan is married with a child. They later have sex, but Nathan ultimately refuses to leave his family to be with Grace. In spite of the lingering tensions between them, the characters come together at the novel's end to watch the comet that Maria discovered years prior pass overhead. The experience particularly moves Thomas.