(this image appears for illustrative purposes only and no attempt is made to supersede any copyright attributed to it)
After Za's father was killed while hunting before he could pass the secret of making fire on, Old Mother became convinced his death was divine punishment for using fire. She feared Za's quest to reclaim the secret would bring disaster on the tribe.
The outsider Kal brought the First Doctor, whom he claimed he had seen making fire, to the tribe. Old Mother knew her son would try to wrest the secret from this stranger and his young companions, provoking the wrath of Orb. After Za ordered the strangers imprisoned in the Cave of Skulls, Old Mother waited until the tribe slept and sneaked away to the cave. She took a stone knife with her. She crawled through a secret entrance into the cave, cut the strangers' bonds and told them to flee into the forest.
Za was furious with his mother for having set them free, but he had no time to deal with her. Za shoved her to the ground and knocked her unconscious, then left with his mate, Hur, to pursue the strangers. When Old Mother awoke, she found herself facing Kal, who was even angrier at what she had done. Kal had hoped to wrest the secret of fire from the strangers and use it to usurp Za's position as leader. Old Mother had cheated him of that opportunity. He killed Old Mother with a stone. (TV: An Unearthly Child)
Old Mother was the first character to die in Doctor Who.