Set in 1955, Dr. Joseph MacNare, a prestigious math and physics professor, submits his experimental psychology paper to the university board. Seeking to request approval for further research on his study, he meets with the board, receiving widespread comments of shock and disapproval. The board agrees that his research, which consists of permanently "strapping a newborn animal onto some kind of frame and tying its legs to control levers" is inhumanly cruel and pointless. Despite almost being forced to resign, Dr. MacNare resolves to conduct his experiment alone.   His experiment would give a rodent the means to behave and think like a human through a robot body. After buying rodents at the pet store, Dr. MacNare and his wife, Alice, begin the study in his home office. For several months, they study the movements of the animals and build a robot for a rat to control its body and speech with its legs. The couple realizes that using a newborn rat would maximize the animal's connection to the robot's body, and, for almost a year, they develop the robot and breed rats to have a handy supply. After finishing the complex robot, they use a newborn rat, Adam, as the test subject. They decide they will take Adam out to give him baths, but only after giving him opiates so that he won't know about it.   Adam successfully adapts to the robot body, learning human movements and pronouncing various syllables. Within several months, he can speak in simple sentences and becomes "consciously aware of himself as Adam." However, Dr. MacNare realizes Adam becomes partly conscious during his baths which causes him to half-remember various sensations and display feelings of disembodiment.   When the night of the university's annual faculty party arrives, the couple leave Adam at home alone with their nine-year-old son, Paul. Dr. MacNare forgets to lock the study door during his hurry. When Paul invites his friends over, one of them accidentally enters the study room while looking for the bathroom. The boys gawk at Adam, trapped in the robot body. They pull Adam out of it, jabbing his frantic body with their fingers. After placing him back in the robot body, Adam attacks the boys, killing two of them. Later, when the MacNares arrive, Adam crushes Dr. MacNare's throat, killing him instantly.

Adam's superstitious feelings were caused by his "strange and unique existence" that led to his violent outburst and killing of Dr. MacNare. When the police and investigators arrive, the coroner kills Adam by slamming him on the office desk.