Summers in Tennessee are unfamiliar to fifteen-year-old Rosella Gill, who spent her life growing up in Oregon. But after a traumatic event with her mother that left their family in shambles, Rosella doesn't plan on seeing her old home again any time soon. Together with her mother and seven-year-old sister Hettie, Rosella finds herself ripped from her old life as they move in with her aunt. Though she doesn't mind Tennessee, the town they live in is rural and stagnant. Things get surprisingly more interesting, however, when she and Hettie come across a well hidden in a nearby forest and a staircase that descends into it. Underneath, they find a moonlit lake and a forest with trees boasting intricate carvings of children's faces. Above it all shines an underground moon, which grows a little fuller every time they return. After meeting a man with a haunted past, Rosella begins to realize that the world is not as innocent as it seems, and it has a particular target: her sister.