Agnes, a girl born with progeria--which causes her body to age ten times faster than normal--doesn't know she has only 100 days to live. Her best friend, Moira, has been her protector from a young age, but now Moira has grown much bigger than Agnes, too big, she thinks. Boone is a boy who used to be friends with both girls--before he did something awful and turned Moira and Agnes against him. Now events conspire to bring the three back together. Romantic feelings spark, and friendship is put to the test.