Arietti, along with her father Potter and her mother, Homilly, are small people from a family of borrowers who live under the floor of an old house in the suburbs.
The borrowers "borrow" the necessities of life, such as sugar, cookies, soap, toilet paper, etc., from human homes without being discovered by humans, and if they are discovered, they will move away immediately.
A teenager named Xiang suffered from a heart attack and moved to the old house where the Arietti family lived to recuperate, and lived with his aunt Maki Sadako and the domestic servant Ah Chun. On the day of the move, Xiang accidentally saw the little man Arietti in the grass and became interested in it.
That night, Arietti follows her father Potter to experience the first "borrowing", but accidentally drops a borrowed sugar cube and is discovered by Xiang. The next day, Xiang places the sugar cube and note in front of the vent, and after Arietti informs her parents of the incident, Potter warns her not to take the sugar cube left by humans. Arietti decides to go to Xiang's room alone to return the sugar cube, only to be saved by Xiang.
Later, the domestic servant Ah Chun discovers the home of the borrower family, and she captures Arietti's mother, Homily, and is more eager to capture the other villains.
Arietti rescues her mother with Xiang's help.
The family leaves the old house in the middle of the night, Arietti and Xiang say a final goodbye, Xiang gives the sugar cube to Arieti, and Arieti gives Xiang the hairpin on her head and thanks Xiang for taking care of this time.
Miyazaki builds a special relationship between a dwarf girl and a human boy, two people who shouldn't have met together, and from the moment they met, they became unstable and wobbly. Compared to the cheerful dwarf, the human boy is lonely, he is sick and has little contact with his family.
Xiang, 12, moved to a mansion to prepare for a summer week in order to recover from the disease. Although dwarves and humans live in the same world, they have always lived in a state where they do not disturb each other. This balance was upset by the boy's pampering. At the same time that the boy's feelings began to change, the quiet life of the dwarf also began to be noisy and collapsed.
One of the themes of the film is the word "borrowed", which is used in the title of the film, where various creatures coexist and prosper together in our world. Animals, insects and plants, initially, life must have no boundaries, you cannot separate yourself from others. There are no human beings in the world, and what animals and plants can fully possess is to live by borrowing everything from nature. In other words, humans and dwarves are parasitic in nature. It is just that in the natural activities of man is not a balancer, often accelerating the imbalance of the natural environment, from ancient times to the present.