The game opens with a paratroopa delivering a letter to Mario's house. Luigi then goes outside, gets the letter, and informs Mario of the delivery. It is an invitation to a party at Princess Peach's Castle, from the princess herself. Mario and Luigi immediately head for the castle and arrive there in no time.
Upon entering the castle Luigi decides to mingle on the first floor for a while and suggests that you see the princess. Then, you will be able to control Mario for the first time. Feel free to explore the controls and the castle and to talk with the diverse cast of party goers. There is no time limit. Head straight up the stairs and through the double doors to reach the next floor.
Eventually, you will reach the princess. She is overjoyed to see you and asks that you accompany her on the balcony. However, the castle suddenly begins to shake and lift from the ground into the starry sky. After the rumbling stops, Bowser and a magikoopa break in through the window. It is then revealed that Bowser lifted Peach's castle into the sky and that her castle is now sitting on top of his. After the princess and Bowser finishing arguing, Bowser will challenge you to fight.
You are only able to attack Bowser by jumping on him. Bowser attacks with his claw and it takes away one HP. After three exchanges, Bowser will pull out the Star Rod (which he stole from Star Haven) and make himself invincible. You will not be able to damage him and his attack power will greatly increase. Soon, you will lose, and Bowser will send you flying to the Mushroom Kingdom below...