While on holiday, Scott Carey is exposed to a cloud of radioactive spray shortly after he accidentally ingests insecticide. The radioactivity acts as a catalyst for the bug spray, causing his body to shrink at a rate of approximately 1/7 of an inch per day.

A few weeks later, Carey can no longer deny the truth: not only is he losing weight, he is also shorter than he was and deduces, to his dismay, that his body will continue to shrink.

Richard Matheson's novel follows through its premise with remorseless logic, with Carey first attempting to continue some kind of normal life and later having left human contact behind, having to survive in a world where insects and spiders are giant adversaries. And even that is only a stage on his journey into the unknown.