After the events of the previous film, the villain Bowser is held in tiny form by Mario and his brother Luigi. From his prison, however, he still sings and yearns for Princess Peach and finds it difficult to control his anger. Moreover, Bowser Jr., his maniacal son, is on the warpath. He wants to get his father back and has to take on the plumber brothers. Meanwhile, Princess Rosalina's good life is also being threatened.