Plot

The Clock family are four-inch-tall people who live anonymously in another family's residence, borrowing simple items to make their home. Life changes for the Clocks when their teenage daughter Arrietty is discovered.

Fourteen-year-old Arrietty (Bridgit Mendler, Saoirse Ronan, and Mirai Shida) and the rest of the Clock family live in peaceful anonymity as they make their own home from items that they borrow from the house's human inhabitants. However, life changes for the Clocks when a human boy discovers Arrietty.—ANN

The Borrowers, tiny creatures living secretly underneath the floorboards and behind the walls of a house, must be extra careful to avoid dangerous encounters with the feared humans. But things take an unexpected turn when adventurous Arrietty, a bright-eyed girl dreaming of proving herself, accidentally meets an ailing human boy. Now, the unwanted attention is putting her family in danger. After all, they are only a few inches tall. Can Arrietty protect their sensitive secret? Will she outsmart suspicious housekeeper Haru, a woman determined to uncover the truth about the little intruders living in the shadows?—Nick Riganas

A boy named Sho (Ryunosuke Kamiki) remembers the week in summer he spent at his mother's childhood home with his maternal great aunt, Sadako (Keiko Takeshita), and the house maid, Haru (Kirin Kiki). When Sho arrives at the house on the first day, he sees a cat, Niya, trying to attack something in the bushes but it soon gives up after it is attacked by a crow.

Grandmother's house is itself spread over a large area, covered with greenery and lots of trees all around. The house has a very classical appearance as it was built in a different era, using wood, bricks and stones. From the appearance it is obvious that the house has not been repaired for a very long time.

Sho then discovers Arrietty (Mirai Shida), a Borrower (and a tiny person, like a fairy but with no magical powers), emerging from the bushes and returning to her home (a whole beautiful world hidden under a cluster of bricks, below Sho's basement) through an underground air vent. Arrietty lives with her father Pod and mother Homily, who restrict her from going outside, afraid that a human might see her and that the world is full of dangerous creatures. Arrietty's great uncle was eaten by a toad. Human children are expected to be more savage than grownups.

Later at night, Arrietty's father, Pod (Tomokazu Miura), takes Arrietty on her first "borrowing" mission above the floorboards to show her how he "borrows" sugar and tissue. Arrietty is already 14 years old, and her father wants her to learn the skills of borrowing to survive on her own. Pod takes his regular route to the rooms above the floorboard, which is made arduous due to their little size.

Basically, their existence depends on "borrowing" stuff from the human world, things like food and clothes. It takes a lot of planning and effort to borrow even a single item as distances, heights and gaps are huge. Pod uses hooks and ropes and double-sided tape to get to where he wants. After obtaining a sugar cube from the kitchen, they walk within a wall to reach a beautifully intricate dollhouse (with working electric lights and kitchen utilities) in Sho's bedroom, to get tissue. However, Arrietty is spotted by Sho while retrieving a piece of tissue from a tissue box and loses the sugar cube. Sho asks her not to leave and although Arrietty hesitates, she still leaves the room with her father.

The next day, Sho leaves the dropped sugar cube beside the underground air vent where he first saw Arrietty. Pod warns Arrietty not to take it because their existence must be kept secret from humans, but his daughter nevertheless sneaks out to visit Sho in his bedroom. She drops the sugar cube he left on the floor, letting him know that she is there. Without showing herself, she tells him to leave her family alone, but they soon have a conversation, which is interrupted by a crow. The crow attacks Arrietty, but Sho saves her.

On her return, Arrietty is intercepted by her father. Realizing they have been detected, Pod and his wife Homily (Shinobu Otake) decide that they must move out of the house. Sho learns from Sadako that some of his ancestors had seen Borrowers in the house, and had the dollhouse custom-built in the hopes that Borrowers would use it. The Borrowers had not been seen since, however, and the dollhouse stayed in Sho's room.

Pod returns injured from a borrowing mission and is helped home by Spiller (Tatsuya Fujiwara), a Borrower boy he met on the way. He informs them that there are other places the Borrowers could move to. While Pod is recovering, Sho removes the floorboard concealing the Borrower household and replaces their kitchen with the kitchen from the dollhouse, in hopes the Borrowers would be more accepting of his knowledge of their existence. However, the Borrowers are frightened by this and instead speed up their moving process.

After Pod recovers, he goes to explore some of the places Spiller has suggested. Arrietty goes to bid farewell to Sho, but in the course of conversation he suggests to her that the Borrowers are becoming extinct. Realizing that he has upset his small friend, Sho reveals he has had a heart condition since birth and will have an operation in a few days. The operation does not have a good chance of success. He believes that there is nothing he can do about it, saying that eventually every living thing dies. Arrietty convinces Sho that he will fight for the life he has now, nevertheless.

Meanwhile, Haru notices the floorboards have been disturbed. Sadako is out and Sho is still in the garden speaking with Arrietty. Haru unearths the Borrowers' house and captures Homily. Alerted by her mother's screams, Arrietty leaves Sho in the garden and goes to investigate. Saddened by her departure, Sho returns to his room.

Haru locks him in and calls a pest removal company to capture the other Borrowers alive. With the help of Sho, Arrietty rescues Homily. Sadako returns soon after the pest removal company's arrival and sends them away. Haru and Sadako discover that the Borrowers have left, and that Sho has destroyed any trace of their presence.

The Borrowers stop for dinner during their move, and Sho's cat, Niya, spots Arrietty. Thereupon Niya leads Sho to the "river", a small rivulet, where the Borrowers are waiting for Spiller to take them further. He gives her a sugar cube and tells her the Borrowers' fight for survival has given him hope to live through the operation. Arrietty gives Sho her hair clip as a token of remembrance, and they go their separate ways. Arrietty, Pod, and Homily leave in a teapot with Spiller. Spiller comforts Arrietty by giving her a red berry after she joins him on the teapot's top.

Sho states that he never saw Arrietty again. Sho returned to the home a year later, indicating that the operation had been successful. He is happy to hear rumors of objects disappearing in his neighbors' homes.