Ernst lived on the planet on which Conrad was king. He was instrumental in helping the Second Doctor and Jamie McCrimmon organise a resistance group to overthrow Conrad's "evil" brother Gavin and put Conrad on the throne.
Ten years later, he entertained the Doctor and Jamie when they "returned" to the planet. Since they had instead been on a doppelgänger planet, they needed information from Ernst and his friend Geffre on what had happened to put Conrad on the throne. (PROSE: Twin Piques)