(103/5F: The Armageddon Factor)
Twin planet of Zeos, located at the edge of the Helical Galaxy, involved in an unending space war against its neighbor. The Atrian Marshal was secretly controlled by the Shadow who desired to obtain Princess Astra, in reality the sixth segment of the Key to Time. Atrios and Zeos were saved from mutual destruction by the Fourth Doctor, Romana and Drax (5F).
- ARMAGEDDON The Armageddon Convention was a galactic treaty which tried to ban the more powerful destruction devices such as the Cyberbombs. But the Cybermen refused to subscribe to it (4D). The Armageddon Factor was the mutual destruction war fought by Atrios and Zeos (5F).
- ASTRA (PRINCESS) Royal princess of Atrios who turned out to be the sixth and final segment of the Key to Time. She was the sixth Princess of the sixth dynasty of the sixth Royal House of Atrios. Astra was kidnapped by the Shadow and later transmuted to her true shape. After the Fourth Doctor defied the Black Guardian, and the White Guardian had used the Key to restore the universal balance, Princess Astra was presumably restored to her human form, returning to a normal life, ruling her planet and presumably marrying Merak (5F). Soon afterwards, Romana decided to adopt Princess Astra's shape when she regenerated (5J).
- DRAX Renegade Time Lord and mechanical genius who had been at the Time Lord Academy at the same time as the Doctor. Drax built the Mentalis computer of Zeos. He was recued from the Shadow by the Fourth Doctor and helped disable Mentalis before the computer could trigger the armageddon factor that would have destroyed both Zeos and Atrios. After the war was over, he helped the Marshal rebuild Atrios (5F).
- HELICAL GALAXY Atrios and Zeos were located at the edge of the Helical galaxy (5F).
- MARSHAL (OF ATRIOS) War chief of Atrios who was secretly under the Shadow's mental command. He led his planet in a fanatical, unending space war against Zeos. He launched a final assault, unaware that it would trigger Mentalis to destroy both planets. The Fourth Doctor placed him in a time loop, and eventually redirected his attack to destroy the Shadow's planetoid. After the war was over, the Marshal helped Drax rebuild Atrios (5F).
- SHADOW Mysterious servant of the Black Guardian who manipulated Atrios and Zeos into an unending space war against each other. The Shadow hoped to capture Princess Astra, in reality the sixth segment of the Key to Time. The Fourth Doctor exposed him and discovered his refuge, a small planetoid located between Atrios and Zeos. The Doctor recovered the Key to Time, then redirected the Atrian Marshal's impending nuclear attack on Zeos against the Shadow's planetoid, which was destroyed (5F).
- ZEONS Inhabitants of Zeos. They had entrusted the conduct of their unending war against Atrios to the Mentalis computer (5F).
- ZEOS Twin planet of Atrios, located at the edge of the Helical Galaxy, involved in an unending space war against its neighbor. Zeos was, in reality, deserted, and the war was waged by the Mentalis computer. Behind it was the Shadow who desired to obtain Princess Astra of Atrios, the sixth segment of the Key to Time. Atrios and Zeos were saved from mutual destruction by the Fourth Doctor, Romana and Drax (5F).