The novel concerns Patricia Lane who is in love with Nicholas Devine, a quiet and gentle writer. Devine undergoes sudden changes, becoming cold and calculating. Frightened by this, Lane consults psychologist Dr. Carl Horker, who rescues her from Devine while under the influence of one of his spells. Devine again attacks Horker, and overcomes him. He is then shot by Lane and rushed to a hospital where a second brain is discovered and removed.