On the surface, the story unfolds through a frame tale: young Kate sits with her aunt Mrs May, crocheting a quilt, while Kate laments missing little items and wonders where they vanish. Mrs May then shares the legend of the Borrowers—a tiny, unseen group of people who live inside houses and quietly borrow everyday objects from the human inhabitants. She recalls how her younger brother befriended a Borrower named Arrietty Clock Aisling Loftus.
Arrietty Clock Aisling Loftus lives with her parents Pod Clock Christopher Eccleston and Homily Clock Sharon Horgan under the floor beneath a grandfather clock. The Borrowers take their surnames from the places they occupy, which in their case is the clock-nest they call home. One day Pod Clock returns shaken from a borrowing expedition, and after Arrietty goes to bed, Pod tells Homily that he has been seen by a human boy who has been sent from India to live with his great-aunt while recovering from an illness. Remembering the fate of their niece Eggletina, who disappeared when a cat was brought into the house, Pod and Homily resolve to tell Arrietty the truth. In a quiet, careful conversation, Homily realizes that Arrietty should be allowed to accompany Pod on borrowing trips.
A few days later, Pod invites Arrietty to join him, and for the first time she is allowed to explore the outdoors—entering the garden through a safe patch of ground beneath the grating. There, she meets the Boy, and after a moment of mutual caution they strike a bargain: the Boy, who is bilingual and slow to learn English, will bring Arrietty books if she will read to him. Arrietty reveals to him that the world may not have enough resources to sustain many humans, a conversation the Boy counters by pointing to the vast population in India alone. The realization that she cannot know whether other Borrowers exist beyond her own family unsettles her. The Boy offers to deliver a letter to a badger sett two fields away where her Uncle Hendreary, Aunt Lupy, and their kin are believed to have emigrated.
The Boy delivers Arrietty’s letter and returns with a cryptic reply asking Arrietty to tell Aunt Lupy to come back. Pod catches Arrietty taking the letter and the family fears expulsion from their hiding place. The Boy soon finds the Clocks’ home and brings gifts of dollhouse furniture from the nursery, initiating a period of extraordinary borrowing. In return, Arrietty is allowed to go outside more and read to him.
Mrs Driver [Victoria Wood] suspects the Boy of stealing after catching him attempting to open a curio cabinet filled with valuable miniatures. One night, her lamp light shines through the floorboards, revealing the Borrowers’ hidden world to her. Believing this is where the Boy has stashed stolen goods, she peeks beneath the boards and is horrified to discover the Borrowers living in their tiny domain. To prevent the Boy from aiding their escape, she locks him in his room until the day he must depart for India. She also hires a rat catcher to fumigate the house in an effort to trap the Borrowers.
The Boy eventually escapes his confinement, and in a desperate effort to help the Borrowers, he breaks through the grating in hopes of creating a real exit. A cab arrives to take him away, and Mrs Driver drags him to the vehicle, leaving the fate of the Borrowers uncertain and their future hanging in the balance.
Time passes, and the Boy’s sister—the young Mrs May—visits the home hoping to prove her brother’s stories true. She leaves small gifts at the badger sett, and later discovers a miniature memoranda book that recounts the Borrowers’ entire tale, presumably penned by Arrietty. When Kate rejoices that the book confirms the Borrowers’ survival, Mrs May points out a striking twist: Arrietty’s handwriting matches Mrs May’s brother’s handwriting, suggesting a connection that blurs the line between legend and memory.
