Hazel Bly used to live in the perfect house with the perfect family in sunny California. But when a kayaking trip goes horribly wrong, Mum is suddenly gone forever and Hazel is left with crippling anxiety and a jagged scar on her face. After Mum's death, Hazel, her other mother, Mama, and her little sister, Peach, need a fresh start. So for the last two years, the Bly girls have lived all over the country, never settling anywhere for more than a few months.

When the family arrives in Rose Harbor, Maine, there's a wildness to the small town that feels like magic. But when Mama runs into an old childhood friend--Claire--suddenly Hazel's tight-knit world is infiltrated. To make it worse, she has a daughter Hazel's age, Lemon, who can't stop rambling on and on about the Rose Maid, a local 150-year-old mermaid myth.

Soon, Hazel finds herself just as obsessed with the Rose Maid as Lemon is--because what if magic were real? What if grief really could change you so much, you weren't even yourself anymore? And what if instead you emerged from the darkness stronger than before?