Princess was walking down the street after a fight with the Powerpuff Girls when she finds a mask. When she puts it on, she becomes a green-headed heroine who later becomes the new Powerpuff Girl, Bucky.
When Blossom and Bubbles are asleep, Buttercup wakes up to see a green glow. Thinking it was Butch, she flies off to see something floating in the river. When she sees it's just a mask, she decides to keep it. When she puts on the mask, she becomes the green-headed Powerpuff Girl, Buttery