Hazel Bly used to have the perfect family. But when a kayaking trip goes horribly wrong, Mum is suddenly gone forever and Hazel is left with a jagged scar on her face. After Mum's death, Hazel, her other mother, Mama, and her little sister Peach needed a fresh start. So for the last two years, the Bly girls have lived all over the country. They travel from town to town, never settling anywhere for more than a few months or so. With Peach running headlong into every little adventure and Mama still heartbroken, it's up to Hazel to keep her family safe.

But when the family comes to the town of Rose Harbor, Maine, a small fog-draped beach town full of roses and rocks, Hazel senses a wildness to it that feels like magic. And when Mama runs into an old childhood friend Claire, suddenly Hazel's tightly-knit world is infiltrated. To make it worse, she has a daughter Hazel's age, Lemon, who can't stop staring at Hazel, clicking photos with her instant camera, and rambling on and on about the Rose Maid, a local myth a hundred and fifty years old about how a local girl was so heartbroken she turned into a mermaid and has lurked in the harbor ever since.

Hazel dives into Lemon's world, one full of lore, mermaid quests, and the eerie story of the Rose Maid, who even more eerily, looks exactly like Hazel. Soon, Hazel finds herself just as obsessed with the Rose Maid as Lemon is-because what if magic were real? What if Hazel could step into the ocean without fear? What if grief really could change you so much, you weren't even yourself anymore? What if instead you emerged from the darkness stronger than before?