Mike Tyson (or Mr. Dream in later versions) is waiting for you at the end of Punch-Out!!
You have to fight a gaggle of other boxers to even get to him. You have passwords to save your progress, but the best one you can get will still stick you with several guys to go through before facing the champ.
That's why I like to put in this password:
This will take you straight to whoever's at the end of your cartridge, Tyson or Dream.
Unfortunately, he's kind of tough. Way tougher than anyone else in the game.
And if you lose, you don't get another chance, you get booted to the game over screen.
Which boots you to the title screen, where you can put the password in again, so no big setback.