Planet (The Exterminator)

An Earth-like planet was home to a Dalek base where they had constructed a giant Exterminator to destroy Earth. The Second Doctor, John and Gillian arrived on the planet and learned what the Daleks were up to. To defeat them, they derailed the train carrying the Daleks who had been trained to operate the Exterminator, stole the weapon's instructions so the Doctor could learn to operate it himself, then used the weapon to destroy all the Daleks. (COMIC: The Exterminator)