Earth has taken Venus as a colony and made its people into slaves. Karl Frantor, a man who disagrees with this state of affairs, visits ancient Venusian ruins with his friend Antil. There they find an ancient tablet and five devices of unknown purpose. After Antil has read the ancient texts, he warns Karl--who is likely to be influential in Earth's government, in the future--that he should exert all his power to improve the treatment of Venusians, or Antil may be forced to destroy the Earth.
Five years pass, and the Venusian situation does not improve. Finally, a small group of apparently unarmed Venusians rapidly liberate all of Venus. Karl warns the government of Earth about Antil's threat, and asks to be sent to negotiate with him.
When Karl arrives on Venus, Antil explains how the planet has been freed of human control: the five devices were weapons that disconnect the mind from the brain, somewhat like decerebration. What's more, decerebrating every human in the captial city--half a million of them--required only a single activation of the weapon. And, worst of all, the weapon is not limited by distance; Antil could destroy every human on Earth in an instant, without leaving Venus.
When Karl conveys this information to the Minister of War, Jan Heersen, Heersen refuses to believe him, and orders an attack. When the attack begins, half the fleet suddenly begins flying out of control. When they are finally subdued, the crew are all found to be mindless husks.
So, Earth surrenders and agrees to Venus's terms: independence from earth and equal treatment, in exchange for free trade. This agreement in place, the Venusians destroy the weapons and the document that explains them by hurling them into the sun.