From the Super Mario Wiki
When the player decides to stop here, s/he will meet Spear Guy, a warrior who is looking for a mustached man to challenge. His quest, Mustached Hero!, involves playing as Mario or Luigi. As one of them, s/he must go to Spear Guy's area, where he will challenge the player to a duel, specifically the Duel minigame Tank-Down. Success will earn the player the right to keep the game.