Beakman explains how light rays reflect off of smooth vs. bumpy surfaces. Since a mirror is smooth and is coated with a metallic backing, the light comes right back at the perfect angle, creating a reflection.
When Cro tries to confront his reflection for stealing a prickly pear from him, Pakka explains to Cro how his reflection is actually him, just flipped!
A boy and his tribe, the Pawnee nation, gets involved in a fight with another tribe. They were successful at the cost of the boy's horse getting shot. Fortunately, the boy finds another horse and celebrates with his tribe as he finds more.