Earth, in the far future: the fearsome Neutron Knights, led by the bloodthirsty warlord Catavolcus, descend on the inner sanctum of the fortress of a king and his few remaining defenders. The king's wizard summons the Doctor and his TARDIS via mental control. The wizard reveals to the Doctor the heart of the inner sanctum, a colossal fission reactor called the Dragon, which the Neutron Knights intend to possess. The wizard sets the Dragon to overload, hoping to consume the enemy in the ensuing explosion, while the king and his men escape in the TARDIS, but the Neutron Knights storm the sanctum.