The trail for the Key to Time is near an end, and there is only one more segment to find. As they have had no dealings with the Black Guardian, the Doctor is warned that this may be when they meet him.
The TARDIS materialises in an underground labyrinth on Atrios. Atrios is at war with a neighbouring planet, Zeos. Casualities are high, and the medical centre is up to full capacity. Princess Astra is always keen to help the victims, and she does not agree with the Marshall.
Every fleet the Atriosans send up always gets destroyed, and Zeos almost seems unharmed. The Doctor says he can help, and says he can build a psychological barrier that would make the Zeons not want to attack Atrios. For that he must have a Zeon prisoner to test. The Marshall says that in war there are no prisoners, but it is strange that no-one recalls seeing a Zeon.
The Black Guardian has a servant, the Shadow, who is on a hidden third planet in between. The Doctor is led to the third planet and encounters an old friend, Drax, who built Mentalis, the Zeon computer, to fight the war.
With Drax's help the Doctor escapes from the Shadow with Romana. Princess Astra is the final segment of the Key to Time, and when the Key is assembled the Doctor has power over ALL time.
The Black Guardian appears as the White Guardian, although the Doctor tricks him into revealing his identity. The Doctor says that the Key is too powerful for any one being, or any Guardian, and orders it to split up, where all the pieces return to their original states.
