In 1992, Edie is at home in London with her mother, Meredith, when a letter that was originally sent during the Second World War arrives in the mail after being lost for decades. Meredith has an emotional reaction to the letter, which surprises Edie and spurs her to investigate its origin. She learns from her mother that as a child, she was evacuated to the countryside during the war and taken in by three sisters living in Milderhurst Castle. Sometime later, Edie is in Milderhurst village for her job as an editor and notices the castle. Intrigued by her mother’s story of evacuation during the war, she finds out from a local that the castle was the home of her favorite author, Raymond Blythe, and that his three daughters remain living there. This prompts Edie to accept a tour of the castle, given by one of the daughters, Percy.
On her tour of the castle, Edie meets Percy and Saffy, the twin daughters from Raymond Blythe’s first marriage. She hears about the youngest sister Juniper, who was abandoned by her fiance in the war and constantly relives that night in a state of dementia. At one point during the tour, Edie is alone in the hallway and comes across Juniper, who mistakes her for her mother Meredith.
Meanwhile, the story cuts back to 1941 when Saffy and Percy are both in their early thirties and preparing for Juniper and her guest to come for dinner at the castle. Saffy comes to the realization that the guest Juniper is bringing is her lover and she suspects the dinner is to announce their engagement. As the sisters prepare for the evening ahead and wait for their guests, a storm begins to brew outside.
The story then moves forward again to 1992 where Edie tries to find out more from Meredith about her time at the castle, but to no avail. She resorts to asking her Aunt Rita, Meredith’s sister, and is given a box of letters Meredith wrote home during the war. The letters reveal that Meredith enjoyed her time at the castle immensely, having developed a close friendship with Juniper, and was extremely reluctant to come home to London, back to her real family. Around this time, her father has a heart attack and Edie moves home to help her mother care for him. She begins reading her favorite book to her father, “The True History of the Mud Man”, by Raymond Blythe and he becomes interested in her investigation into the story’s origin and the Blythe sisters. Meanwhile, her mother is furious to find that Edie has read her letters and the two have a strained relationship until her father coaxes them to patch things up. This leads to a reconciliation between mother and daughter and Edie gets a better understanding of her mother and why she was so emotional upon reading the lost letter from Juniper. Not long after, Edie is contacted by a publisher stating that Percy Blythe had requested Edie to write the introduction for the new, anniversary edition of “The Mud Man”.
Back in 1941, tensions rise throughout the day as the sisters prepare for Juniper’s wedding. It is revealed that Percy and the former housekeeper Lucy had been in love, but because of the time they live in, they could never have been together romantically. It is also revealed that Saffy desperately wishes to leave the confines of the castle but has up until now, never had the courage to leave. Juniper has been living for several months in London and has fallen in love with Meredith’s former teacher, Thomas Cavill. Up until her father's recent death, Juniper has always been kept sheltered within the castle walls. She was always extremely creative, which her father tried to foster. She also suffered from violent spells where she would black out and not remember what she had done. In London, away from the castle, Juniper finally feels free and has none of her usual blackout symptoms. She is madly in love with Thomas and they wish to marry but Juniper is hesitant at first. She is aware that her recently deceased father made a will that left the castle to her on the condition that she never marry. If she marries, the castle will be given to charity and her twin sisters will be forced out of their ancestral home. Despite all this, she eventually agrees to Thomas’ proposal because they both cannot bear to be without each other. She plans on telling the twins of their engagement when she brings Thomas home for dinner.
In 1992, Edie arrives at the castle to interview the twins and learn more about the castle and its secrets. Percy sits her down to finally tell someone the truth of what happened the night Juniper’s fiance abandoned her and the origins of “The Mud Man”. When her and Saffy were children, their mother was ill for a long time after their births and because of this, they grew much closer with their father. Eventually, their mother regained her health but felt left out of the family and so she took a lover named Oliver Sykes, an architect working on redesigning the castle’s gardens. Raymond discovers them one night and is so enraged he starts a fight Oliver, which leads to a fire starting in the room and his wife’s dress immediately going up in flames. Oliver also catches on fire and tries to jump out of the window into the moat below where he drowns. Saffy, a young child, witnesses him jumping from the window below her and trying to claw his way out of the muddy moat. This trauma leaves her with significant emotional damage, which takes the form of recurring nightmares about a ‘mud man’ that tries to climb the castle walls. Years later, when Raymond returns from World War I, he has his own post-traumatic stress to deal with and has trouble with nightmares and sleeping. Saffy ends up confiding in him her recurring nightmare, which he then turns into his famous and hugely successful novel “The True History of the Mud Man”. Saffy’s trauma is made worse by the fame and success of the book and she recedes further into her life at the castle, causing her to lose her fiance who promptly marries her cousin instead.
After explaining the origin of the ‘Mud Man’ story, Percy explains what happened the night Juniper and Thomas were meant to come for dinner. The storm that night became very violent and due to the wartime blackouts, the countryside around the castle was pitch dark. Only a small sliver of light was able to escape from the castle’s parlour because the shutter was slightly broken. Juniper arrives at the castle first and is immediately distraught that she has blood on her dress. She is afraid she has had one of her spells where she does something violent and forgets, believing she may have harmed Thomas in some way. Saffy calms her down and puts her straight to bed, giving her a sleep aid. Saffy is also on edge, so she decides to take some of the sleeping aid she had given Juniper and sinks into a deep sleep in her chair in the parlour. Meanwhile, Percy decides to go outside in the storm and look for Thomas. She sees the sliver of light coming from the parlour and decides to go fix the shutter when she notices that Thomas is already over there. As Thomas climbs the wall to peek in the window, Saffy is having her recurring nightmare about the Mud Man and wakes to see Thomas, covered in mud from the storm, looking in the window. Saffy gets up and strikes him with the wrench that had been left on the windowsill to repair the shutters earlier that day. Thomas falls back and dies with Saffy never realizing she had actually killed him and that it wasn’t a dream. Percy, having witnessed the whole thing, buries him and decides to keep his fate from both her sisters so that neither would be hurt. It is later discovered that the blood on Juniper’s dress was actually from when she stopped on the roadside in the village to help deliver a baby. However, she always believed deep down that she might have had something to do with Thomas’ disappearance and lives the rest of her life in a demented state, believing it is that day in 1941.
Once Percy finishes with the story, Edie leaves and decides not to share it with the world. Percy is content knowing that Edie will let Thomas’ family know that he didn’t abandon them and actually died in an accident decades earlier. Percy decides to set both her and her tormented sisters free by setting fire to the castle and the three of them perish. Meredith is also at peace knowing that Juniper never actually forgot about her. Edie and Meredith finally have a closer relationship, having been brought together by their shared experiences with Milderhurst Castle and the Blythes.
