When Libby was young she was deemed the 'fattest girl in the world' when she had to be lifted from her house with a crane because of her immobility. She faced this for many years before finally going back to school.
Jack struggles from a rare disease called prosopagnosia which is basically causes him to not be able to recognize faces of anyone including close family and friends. He has kept the secret his whole life from everyone including his parents.
Libby and Jack get through the school year together and tell each other their secrets they had been keeping from everyone. Libby becomes an advocate for other girls going through what she was going through and Jack finally tells everyone about his disease. They end up falling in love because of the support they gave to each other. In the end they held up each others universes.