Pluto follows the Europol robot detective Gesicht in his attempts to solve the case of a string of robot and human deaths around the world where all the victims have objects shoved into or positioned by their heads, imitating horns. The case becomes more puzzling when evidence suggests a robot is responsible for the murders, which would make it the first time a robot has killed a human in eight years. All seven of the great robots of the world, the most scientifically advanced which have the potential to become weapons of mass destruction, seem to be the killer's targets, and the murdered humans are connected to preserving the International Robot Laws which grant robots equal rights.