Seventh-grader Millie has the best friends in the whole world: Florence, Claire, Gabby, and Rosie, but when Rosie dies in a car accident everything changes. Rocked by grief, the remaining four girls struggle to move on. Millie barely understands her normal pre-teen feelings, let alone the messy ones left behind by Rosie, so she outruns her emotions by throwing herself into a mystery: a cryptic notebook abandoned at her family's laundromat. Could the clues in the notebook be related to Rosie?

Together, Millie and her friends embark on a heartwarming journey to heal from the loss of Rosie and end up finding more than they ever could have even imagined.