Arrietty Clock is a head-strong teenage Borrower whose parents, Pod and Homily, are terrified of the 'Human Beans' who live above them. Desperate for freedom and adventure, Arrietty sneaks out one night and befriends a lonely boy who lives upstairs. But Arrietty's adventure inadvertently reveals her family's existence to the scheming scientist Professor Mildeye, forcing them to go on the run into the big bad world.

A family of miniature people who live beneath the floor of a country house, borrowing things they need from the humans, are forced to leave their home when their teen-aged daughter, Arietty, is seen by a human.