Emma Hope Arnett thought her days as a big sister would be filled with baby drool and tummy time: in other words, so much fun, she would want to skip Runners' Club and spend every moment with her now-bigger family.

Instead, she can't wait to get out of her house each morning, away from a sad mom and a stressed dad.

What could have gone so wrong?

Her mom is sick, that's what. She has postpartum depression. But Emma and baby Charlie still have a full life to lead. When Runners' Club, Emma's favorite escape, gets canceled, Emma is devastated. Can Emma convince her principal to restart Runners' Club, and can she solve her family's problems, or are some problems too big for a third grader to solve?