The Guardians were physical embodiments of good and evil, light and dark, calm and chaos. The White Guardian foresaw that the Universe was on the verge of descending into the chaos that his opposite, the Black Guardian craved. He summoned the Doctor, setting him on a quest to find the Key to Time, a device separated into six disguised segments. Once placed together, the Key would literally stop the Universe for a few seconds, enabling the White Guardian to restore the balance.

When the Doctor finally assembled the Key, the Black Guardian pretended to be his opposite and asked for the Key. However, his callous disregard for the life of the final segment, Princess Astra, convinced the Doctor that he was the wrong Guardian and so he dispersed the Key again, earning the Black Guardian's eternal hatred. The Black Guardian later placed Turlough aboard the TARDIS with orders to kill the Doctor, but eventually he rejected the Black Guardian, who was expelled from the Universe. However, the Black Guardian was not destroyed as the White Guardian still existed, dark always necessary to balance the light.

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