Few things feel worse than being left behind by your best friends, but that's what's happening to thirteen-year-old Dani. She has far less freedom than they do when it comes to roaming their New York City neighborhood, thanks to her over-protective mom. Dani knows her mom has her reasons--including having lost her leg in an accident when she was Dani's age. But it still doesn't seem fair. Following an especially painful falling out with her friends, Dani finds herself wishing she could change her mom's past, making life easier for both of them. Soon after, she wanders into an oddity shop where she discovers a letter in an old book. It's from a girl named Mimi who is seeking a friend. The two begin leaving daily letters for each other in the book, growing close and connecting over losing their best friends.

Some of Mimi's sayings and experiences are very dated, but Dani manages to explain this away--until she can't ignore the strange coincidences and odd references. Who is her mysterious new friend? Do her letters hold the clues Dani needs to heal her broken relationships, especially with her own mother? Or is she too late?.