Eminent scientist Emil Keller has developed a revolutionary new process for the treatment of hardened criminals. His invention, the Keller Machine, is being heralded as a major scientific breakthrough. But Professor Keller is in truth the Master and the Keller Machine is much more than a mere machine. Soon the Doctor is involved in a bitter struggle with his deadliest enemy, an alien mind parasite, and a diabolical scheme to plunge the world into a Third World War...