A young boy's behavior shifts drastically when he begins communicating with an unseen presence that challenges his understanding of reality. Matthew, an eleven-year-old, finds himself under the influence of an entity named Chocky, which manifests as an invisible companion. His parents observe with growing concern as Matthew displays knowledge beyond his years and exhibits erratic behavior. The narrative unfolds through a first-person perspective, documenting the mounting tension as external forces begin to investigate the nature of this connection and its potential implications for humanity.

Readers frequently highlight the understated, grounded approach Wyndham takes toward the science fiction premise. Discussion often centers on the unsettling nature of the possession-like dynamic between the boy and the entity. Critics often point to the effective pacing that shifts from a domestic drama to a tense thriller as the scope of the mystery expands. The book is noted for its ability to maintain a sense of unease without relying on overt action sequences. Many readers appreciate how the story prioritizes the psychological impact of the contact over the mechanics of the alien presence itself.