A symbiotic alien investigator tracks a dangerous fugitive across the unfamiliar terrain of Earth, forced to inhabit a human host to survive and complete his mission. The protagonist, Hunter, is a law enforcement officer from a gelatinous, shape-shifting species who must navigate the complexities of human biology and social structures. His objective is to locate and neutralize a criminal of his own kind who has also taken refuge within a human body. The narrative follows a first-person perspective, detailing the logical constraints of the alien's physiology and the challenges of operating in a world where he is essentially a parasite, albeit a benevolent one, within a teenage boy.

This work is widely regarded as a foundational example of hard science fiction, noted for its rigorous attention to biological detail and logical consistency. Readers often highlight the ingenuity of the symbiotic premise, which allows for a unique perspective on human behavior and environmental interaction. The narrative is frequently praised for its disciplined pacing and the intellectual satisfaction derived from the protagonist's methodical approach to his investigation. While the prose is functional and direct, the strength of the world-building ensures that the story remains a significant entry in the genre's history.