Big things are about to happen at Maple's house. Mama's going to have a baby, which means now there will be four Rittle sisters instead of just three. But when baby Lily is born too early and can't come home from the hospital, Maple knows it's up to her to save her sister. So she and sister Dawn, armed with a map and some leftover dinner, head off down a river and up a mountain to find the Wise Woman, who guards a pool with miraculous powers. But the dangers Maple and Dawn encounter on their journey make them realize a thing or two about miracles--and about each other.