It is set in a centuries-post-apocalypse rural area whose leaders are frantic to keep God from bringing Tribulation upon the land again. They believe that in order to keep this wrath at bay, they need to root out mutations. Most of the time, these mutations manifest as odd plants or animals. But on occasion, they occur in humans, and those human mutants are banished and sterilised. Into this setting are born the protagonist, some of his relatives, and a few scattered others: Their mutation cannot be seen, making them all the more dangerous. They are telepathic. Wyndham tells this story with his characteristic dry humor and a protagonist that wisely fails to take himself too seriously.
John Wyndham prefered the title 'The Chryslids' for this book. The word chrysalid means "relating to or resembling a chrysalis". This title is perhaps ironic: a chrysalis being a "protected stage of development" in the life cycle of a moth or butterfly.