A long, long time ago, a girl lived with her tribe in a cave. As she had no parents of her own, her pet cave bear was her best friend.

He followed her everywhere and comforted her whenever she grew weary of all the chores she was given. So when Nell overhears some of the grown-ups planning to give Cave Bear to another tribe, she decides they must run away. So together they embark on a great adventure, following the stream from their cave down the mountain to wherever it will lead them.

Together they encounter huge mammoths and wild hunters, face hunger and thirst, all the time hoping to find somewhere safe to be together.