Bowser is bored, and decides to use up his time by attempting to kidnap Peach. However, his castle is at the other side of the Mushroom Kingdom so he has to traverse all the way there and back safely.
The basic tutorial level, in the end of it, the Koopa Clown Car breaks.
Another tutorial level, Bowser still has his fire breath. but after this, a hyper goomba dashes past bowser and making him going high into the sky but still finishing the level.
Your first non-tutorial level, Now you encounter some toads which are like goombas, but take 2 hits to kill. Also, Bowser gets to use a stronger stomp attack.
This world starts out with some toads, and then goes to the first boss, the Toomba, which is a fusion of a toad and a goomba. After defeating it, Bowser goes to world 2.