You start in World 8-4 from the original game, but as you continue it switches to a portal-themed level. The level continues switching between the original 8-4 and portal-styled areas. When you get to the end you fall into a confined room and you battle Bowser.
When you defeat him, he explodes (in megaman style) and the game fades to black. It then fades back in and you see Glados. You then have a battle with her. Upon defeating her, you win the game and save Peach.
(Thanks idiot9.0 for the translation)
(Sorry for the bad thread before)