A young boy named Shawn moves into a countryside house to rest before an upcoming heart surgery. Unknown to him, beneath the floorboards lives a family of tiny people called Borrowers, who survive by "borrowing" items from the human household. On one of her missions to collect supplies, 14-year-old Arrietty, a bold and curious Borrower, is spotted by Shawn. This unexpected encounter threatens the secrecy of her family's existence. As Shawn tries to befriend Arrietty, their worlds collide, evoking a mix of curiosity and caution.

Arrietty's parents, Pod and Homily, worry about the dangers of being discovered by humans. Despite their warnings, Arrietty forms a bond with Shawn, effectively learning more about the human world above. However, the Borrowers' existence becomes perilous as Haru, the housekeeper, becomes suspicious of their presence. The family's safety is compromised as Haru sets out to capture them. Struggling with the consequences of this friendship, Arrietty and her family must decide whether to flee or face the uncertainties brought by their exposure to humans.