Federico Guillermo, a 7 year-old boy, gets grounded for making a mess while playing in the living room and not wearing the orthopedic shoes he's been told. Federico refuses to wear them but he just can't take them off so he decides to run away from home. His sister is just able to yell at him: 'tie your laces or you will fall!!'. In the run Federico suddenly falls down and discovers that he fell because of a fabric hole on the pavement and not for the laces. He tries to set free his foot from the threads, realizing that every time he pulls a string something is destroyed in the world. Federico starts looking for the string that will destroy his own shoes.

In the small town of Rockwell, Maine in October 1957, a giant metal machine befriends a nine-year-old boy and ultimately finds its humanity by unselfishly saving people from their own fears and prejudices.