The novel takes the form of an oral history told by a young man named "Rush that Speaks" and of his wandering through a strange, post-apocalyptic world in pursuit of several seemingly incompatible goals.

Rush comes of age in Little Belaire, a mazelike village of invisible, shifting boundaries, of secret paths and meandering stories and antique bric-a-brac carefully preserved in carved chests.

Over the centuries, the people of Little Belaire have perfected an art which they call "truthful speaking": communication so clear and accurate, so "transparent", as to leave no potential for deception or misunderstanding.

Rush's journey is set in motion when the girl he loves, Once a Day, elopes from Little Belaire to join another group, an enigmatic society called Dr. Boots's List.

Writing in the New York Times, Frederik Pohl praised Engine Summer, noting, "Mr. Crowley presents his society from the inside, yet his point of view is off‐center, so that the reader learns slowly and never feels secure enough in his knowledge to reduce everything to a glib formula or two.