The story begins when Old Ezra Kiles, a resident of Malville, becomes unusually fixated on the topic of evolution. His obsession manifests during a circus visit when he encounters a baboon in a steel cage. Ezra, convinced that the baboon is a close relative to humans, demands the circus manager release the animal, arguing that the baboon should be free to enjoy the blessings of civilization and pursue its own destiny. Despite Ezra's impassioned plea, the manager refuses, leading Ezra to approach the cage and speak to the baboon as if it were human.

He asks whether the baboon's ego is suffering behind the bars. In response, the baboon grabs Ezra by the beard and repeatedly slams his face into the cage bars. This violent interaction is witnessed by Nub Smalls, the village humorist, who sarcastically remarks that the baboon's ego seems to be functioning just fine. This angers Ezra, who then attacks Nub, beating him until others manage to pull him away.

After the incident, Ezra continues his rants, quoting notable figures such as Darwin, Huxley, and Haeckel to support his views. Most people in town assume that Ezra is drunk, so they detain him in the sheriff's office for the night, as the jail is undergoing repairs. When the sheriff arrives, he finds Ezra behaving in an even more bizarre manner. Ezra, now naked, is hopping around the office on all fours and attempting to hang from the chandelier by his toes.

He proceeds to chase the sheriff around the room, nearly cornering him before bystanders intervene, attracted by the sheriff's cries for help. The townsfolk manage to calm Ezra down by offering him coconuts, then tie him up to prevent further chaos. By the next morning, Ezra seems to have returned to his usual eccentric self, and the townspeople release him without further incident.