Dr. Cranley is sentenced to exile in space for murdering a rival scientist, and swears revenge... Later, across the world people suddenly start behaving like lunatics, culminating in their being gripped by hysterical laughter until they collapse-dead. It becomes known as the Laughing Plague. Eventually it is learned that the Earth has crossed the path of a spatial cloud of gas that was the result of an ancient cataclysm that blew the one-time moon of Venus to pieces, leaving its poison atmosphere free in space. Then, radio messages are received from Dr. Cranley, now domiciled and living on Venus. He offers to send a Venusian antidote to save humanity. But can he be trusted?