Princess Toadstool has been kidnapped (again) by Bowzer, the Koopa King. This time, he intends to force her into matrimony, but she's not cooperating...
Mario's ticked off and makes Bowzer crash into the wedding cake, and the Yoshis start to eat the cake. Bowzer tunnels to the top to escape, but Mario is waiting, says something heroish, and the cake collapses. Blah blah blah, good guys win, chaste kiss from Princess, the end.