The book was notable in that it broke new ground in the science fiction field by postulating an alien lifeform, not hostile, which could live within the human body.

The Hunter, an alien lifeform (when not inside another being, resembling a four-pound green jellyfish) with the ability to live in symbiosis with and within another creature, is in hot pursuit of another of his kind.

By the time it has figured out enough of what goes on inside a human being to look through Bob's eyes, it is shocked to find itself within an air vessel, being carried further away from its quarry every second.

As it happens, Bob is simply returning to a New England boarding school from his home on an industrial island in the Western Pacific.

The puzzle distracts Bob from his studies, leading to a decline in grades that the school authorities ascribe to homesickness, and he is sent home for the remainder of the term.

The fugitive alien, fearful of being killed in the explosion, knocks out his host and removes himself from Mr. Kinnaird's unconscious body.

Despite faulting the plotting as "thin" and having the antagonist act "improbably stupid," he concluded the novel was "a good sound entertaining yarn.