Professor Emil Keller has created a machine that can pacify even the most dangerous of criminals. But when the Doctor and Jo arrive at Stangmoor Prison for a demonstration, things start to go horribly wrong - especially when they discover that the Doctor's old enemy the Master is responsible for the machine.
The Master, an old adversary of the Doctor, has arrived on Earth to destroy humanity with the aid of the Nestene Consciousness and their army of Autons and killer plastic daffodils - unless the Doctor, joined by new assistant Jo Grant, can stop them.
