"A teen girl suffers from progeria, a rare disease that causes her to age rapidly. This is the story of three unlikely friends learning to live life to its fullest before ultimately letting it go"-- Provided by publisher.

Agnes doesn't know she only has one hundred days left to live. As a baby she was diagnosed with progeria, a rare disease that causes her body to age at roughly ten times the normal rate, and Agnes has already exceeded her life expectancy. Moira has been Agnes's best friend and protector since they were in elementary school, and Boone was friends with both girls before he did the thing that turned Agnes and Moira against him, before his dad died, before his mom got too sad to leave the house. When the three are brought together and romantic feelings start to develop, and their friendship is put to the test.