On the outside, Faith Sunderly is a typical, well-behaved 14-year-old girl of the Victorian era. Inside, however, she longs to be allowed to read, learn and explore like her father, the Reverend Erasmus Sunderly, a renowned naturalist.
Faith and her family, including her younger brother, Howard; her mother, Myrtle; and her Uncle Miles, are forced to leave London with her father due to a scandal surrounding the publication of her father’s recent findings. The reverend takes a position on the small island of Vane in order to help with an archeological dig. When Faith arrives at their new home, Jane Vellet, the housekeeper, greets her. Faith’s bedroom is the smallest, but she is happy as it is by the servants’ staircase so she can hear their gossip and even sneak out at night.
The following day, Faith goes out to play with Howard and is surprised to see a stranger wandering near the Folly, an old tower where her father has put some of his specimens. Although Mrs. Vellet assures the reverend that the stranger was probably one of the local men using the easiest path to the beach, the reverend insists he will set rabbit traps to stop trespassers.
Later that day, Dr. Jacklers escorts the reverend and his family to the excavation site. He introduces them to other members of the team: Mr. Clay, the island’s curate; Mr. Lambent, who owns the land; and Ben Crock, the foreman. At tea that afternoon, Faith and her mother meet Mrs. Lambent, who frequently observes the work at the site.
That night, a boy is injured in one of the reverend’s rabbit traps. At the time, her father is in his study and has demanded that he not be disturbed. Faith enters her father’s study to ask if they might call a doctor. She is terrified to find him in what appears to be a drugged state. He orders her to leave. She lies and tells her mother that he said they would pay for a doctor to tend to the boy.
The following day, Faith and her mother visit the village and are distressed when the shopkeepers snub them. Her mother stops by the church to see if Mr. Clay has heard anything. Faith is ushered away by his son, Paul, a boy about her age, so that the adults may talk privately. Paul and Faith spar verbally as he is a friend of the boy who was hurt in her father’s trap.
When she and her mother return home, they find the reverend and Uncle Miles have been banned from the excavation site. News that the reverend has been falsifying his archeological findings has reached Vane, and the family is being ostracized. Meanwhile, the reverend has discovered a fingerprint on one of his letters. Furious, he accuses the maid of spying on him and fires her.
That night, Faith admits that she, not the maid, read the letter. Her father scolds and humiliates her but then asks for her help. She helps him take a rowboat out to a cave where he deposits a cloth-shrouded pot from the Folly. When they return home, he sends her to bed and orders her to keep their secret.
When Mrs. Vellet discovers that the reverend’s boots and overcoat are missing in the morning, the house is searched. Faith fears he may have gotten caught in one of the traps. She sees his body draped over the limb of a tree, halfway down the side of a cliff. Although distressed, her mother has the forethought to have Faith hide the reverend’s letters. Faith believes someone murdered her father but no one pays attention to her claim. Myrtle tries to have her husband’s body buried in the churchyard, but it is rumored that the reverend committed suicide, so a full inquiry must be made.
Furious at the callousness of the townspeople, Faith returns home. In her room, she reads through her father’s journal. She learns the plant they hid is a “Mendacity Tree.” A naturalist named Winterbourne told the reverend of its powers. If you speak a lie to it and then circulate the untruth widely, it bears fruit. The greater the lie and the more people who believe it, the bigger the fruit grows. When eaten, the fruit has the ability to grant secret knowledge close to a person’s heart.
The reverend visited Winterbourne in jail to learn the location of the plant and promised to try and have Winterbourne released, but he died of malaria in his cell. The reverend found the Mendacity Tree and experimented by telling it a small lie about his health. When others believed it, the tree sprouted a small fruit that when eaten, revealed the reverend’s future malady of gout. Convinced of its power, the reverend longed to know the truth about Creation, so he knew his next lie would have to be fantastical. He began falsifying his fossils.
After reading the journal, Faith believes her father was murdered for the Mendacity Tree. She sneaks out of the house and finds the tree in the cave. She whispers the lie that her father’s ghost haunts the house, seeking revenge on those who wronged him. She returns to the house and sets up several tricks to create the illusion of a ghost that the servants soon believe.
Faith discovers Paul Clay lurking on the stairs and learns he was dared by his friends to snatch a lock of the reverend’s hair, as the whole island now believes his ghost haunts the house. Faith tells Paul she suspects her father had gone to meet someone the night he died because he took a pistol. Paul agrees to help her find clues in exchange for a lock of her father’s hair.
Knowing that her lie has spread, Faith returns to the cave and finds the plant thriving and bearing a tiny fruit. Eating it, Faith has a strange vision that suggests her Uncle Miles orchestrated their move to Vane in the hopes that he, too, might be respected by the archeologists. The dream also hints that Faith was correct in believing her father was murdered.
She then tells the tree that the leaders of the excavation are really seeking buried treasure. She leaves fake clues to propagate her lie. In order that she might spy on the site, she asks the men if she might sketch some of their findings for scientific journals. She soon learns that villagers are raiding the site at night, looking for treasure. Excited, she returns to find the tree has outgrown its pot, and its vines are spreading across the cave. This time, the fruit shows her a strange vision of museum dinosaurs coming to life and attacking the patrons, including herself.
Faith wakes up and realizes she has walked in her sleep. Paul had dared her to come to a ratting, and she, still under the influence of the fruit, goes to the barbaric event. On her way home, two local boys question her about the treasure. Pretending to be crazy, she implies Mr. Lambert gave a box to a woman. They suggest it was Miss Hunter, the postmistress. Since Miss Hunter had gossiped about her father, Faith agrees. She runs from the boys and returns to the tree, which has grown even bigger. She tells the tree that Mr. Lambert gave the treasure to Miss Hunter. The next day, villagers burn Miss Hunter’s home, looking for the treasure.
Faith discovers her Uncle Miles ransacking her father’s study. Confronted, he claims he has found them a benefactor on the island who will pay generously for the reverend’s papers and plants. Faith refuses to give them up, and Miles attacks Faith until her mother fights him away. Miles washes his hands of their family. In a fit of despair, her mother explains how they are now ruined. The inquest is in the morning and will prove the reverend committed suicide, the family is destitute, and as a woman, she has no hopes of securing a future for them.
Faith returns to the tree, which now inhabits the entire cave and is surprised to find that Paul has followed her there. She decides to trust him, as she must eat its fruit once more. She sees her father visiting Winterbourne in his jail cell and learns her father never intended to free him. He used him to find the Mendacity Tree. When she wakes from the vision, she has Paul take her to the church so she can read the parish registry. In it she discovers that Winterbourne’s widow married Mr. Lambent. She suspects Mrs. Lambent of killing her father in revenge for her husband’s death.
The following day, Faith confronts Mrs. Lambent with her suspicions that she and Mr. Lambent killed her father. Faith is shocked when Mrs. Lambent calls for Mr. Crock, the site foreman. He was the Winterbournes’ trusted foreman and is indebted to his widow. Faith flees home to warn her family they must leave before Mr. Crock finds them. Her family escapes, but Faith is captured by Mr. Crock. He forces Faith to take him and Mrs. Lambent to the tree.
In the cave, Faith shoots Mr. Crock with her father’s pistol and sets the tree on fire, in order to escape. Mrs. Lambent pursues her so Faith pretends she has the tree’s last fruit. Hoping only to distract her, Faith throws a pebble over the cliff. Even after Faith admits she is lying, Mrs. Lambent willingly jumps after it to her death.
With the reverend’s death now ruled a murder, the family is free to bury him and go back to London. Faith leaves Vane, determined to become a naturalist in her own right, even though it will shake the norms of Victorian society.