Bluebell has finally come of age to receive her wings. She turns into her caterpillar form, and weaves a chrysalis, and the twins wait for her to finish transforming. However, the Trolls invade the valley, and the twins must keep Blubell hidden in their world, where more challenges await, as they assume their parents will make them get rid of it.
Eventually, their parents do discover that they are keeping a crysalis, and tell the two they can keep it, so long as they release the butterfly later.
The Twins happily agree.
Blubell emerges from her crysalis the next day, and the two take her back to Butterfly Valley, where Blubell admires her new wings.