Oswald (Children of the Evil Eye)

Oswald was a Human genius from the planet Earth, with an IQ so high that it could not be measured. The government created a special teaching unit for him, and he later produced many amazing inventions. Oswald came to believe that the world would be a better place with children in charge and, using a device he called "the Eye", seized power. The children, whose youth was arrested by Oswald, controlled the world, and the surviving adults (called "Ancients") were enslaved.

The Time Lords sent the Doctor to this time to rectify the situation. When Oswald tried to use the Eye to interrogate him and force him to reveal the secrets of his TARDIS, the Doctor managed to resist, which caused the machine to break down. The Doctor then convinced the Ancients to rebel, and took Oswald with him in the TARDIS to drop him off on a planet where his IQ would only be average. (TVC: "Children of the Evil Eye")