An alien visitor on the Enterprise has a hidden agenda for his own species.
The Enterprise sends a landing party to a planet, along with an observer named Bem, who comes from another world and who is a different kind of being. He has a plan of his own and seems to be hiding something, and Kirk begins to wonder about his intentions.
The story is a first contact tale about observation and trust. It shows Bem's attempt to learn about humans by living with them, and the problems which his actions cause. Fans remember the unusual alien, who is a mix of humour and menace, and the story's questions about how to work with strangers.