The spaceship 'Ariadne' had fought her last fight. The men aboard her were expecting to die. Then a voice spoke out of sapce. The speaker said that he was Hansekar, the overlord of the planet Canistra and that he wished to help them. To that end, he transmitted a radio beam and the Ariadne homed in on it, crashing in the forest of the tree ferns. Presently Hansekar and his retinue appeared on the scene. They were robots all, very ancient robots, the sole survivors of a once great civilisation. And yet there was a mystery about them, for it soon proved that they represented a level of technology which even the Canistrans had never reached. Then came the hour when Captain John Rogers had to ask himself whether Hansekar was more than a robot, and just what kind of secret he kept hidden in the basement of his palace...