"The Girl Who Drank the Moon" sees the people of a land called the Protectorate leave a baby as an offering to a witch who lives in the forest in the hope that the sacrifice will prevent her from terrorizing their town. But the catch is, she's a kind witch called Xan, who shares her home with a swamp monster named Glerk and a tiny dragon named Fyrian.

When the townspeople abandon the babies, Xan rescues them and re-homes them with loving families on the other side of the forest. As she travels through the forest with them, she nourishes them with starlight until one night she accidentally feeds a baby moonlight, which fills the child with extraordinary magic. Xan decides to keep this baby and name her Luna. But as the girl's 13th birthday approaches, her magic begins to surface with potentially dangerous consequences.