Many thousands of years ago strange crystalline creatures came down from the stars and settled on the planet of the Gonds. Over the years they educated the Gonds through teaching machines in the great Hall of Learning. In return, the Gonds periodically selected their two most brilliant scholars to become the 'companions' of these mysterious beings. But when the Doctor, Jamie and Zoe arrive on the planet, they soon discover the true evil purpose of the aliens and learn what it really means to be companions of the Krotons...