The Doctor and his companion "Felicity" arrive on the planet Mondas, Earth's twin orbiting on the opposite side of the Sun. While the Doctor works on a piece of TARDIS equipment, Felicity encounters the gentle Prince Sylvan. Sylvan accidentally activates the TARDIS, sending him, the Doctor and Felicity fifty years into the future. There, Sylvan's brother, Dega, is now king and has used the Doctor's device to begin turning his people into Cybermen. He has constructed a space fleet with which he intends to invade the mineral-rich Earth, and plans to kill any unconverted Mondans with cyanide gas. Felicity appeals to Dega's partly-Cybernised wife, Queen Meta, and she shoots her husband dead -- only to be killed by Dega's chief of staff, Krail. In the confusion, Sylvan and a band of Mondan rebe