A man named Olivero discovers a mysterious woman with green skin, leading him to abandon his life and follow her into a hidden, subterranean realm. Olivero, a man disillusioned with the surface world, encounters the titular green child, who has escaped a life of confinement. Driven by a desire for meaning and escape, he follows her into a millstream that serves as a gateway to an underground civilization. The narrative explores the physical and metaphysical transition from the known world to a strange, static society where the inhabitants exist in a state of perpetual, meditative calm. The story is presented through a third-person perspective that emphasizes the contrast between the chaotic surface and the ordered, alien depths.

Discussion often centers on the book's unique blend of fantasy and philosophical inquiry, which sets it apart from conventional genre fiction. Readers frequently highlight the dreamlike quality of the prose and the author's ability to create a sense of profound detachment from the physical world. Critics often note that the narrative prioritizes thematic exploration over traditional plot progression, which may challenge those seeking a fast-paced experience. The work is widely regarded as a singular contribution to mid-twentieth-century literature, valued for its intellectual depth and atmospheric consistency.