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