A group of gold-skinned aliens known as the Axons land on Earth and offer wondrous technology in exchange for fuel. The Doctor, however, isn't fooled, uncovering the Axons' true nature and once again facing his archenemy the Master...

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.