In the scene where Bowser Jr. is confronting Princess Rosalina, she starts to gain the upper hand. Then a wandering little Luma sticks its head out of the house, curious, so Bowser Jr. attacks the house. Rosalina manages to protect everyone and is defeating Bowser Jr. until you appear and defeat her—because you’re very powerful. Now you’re on Bowser’s planet and the princess is imprisoned.