an adventure mini-game and part of the StreetPass Mii Plaza for the 3DS. It is known as 'Streetpass Quest' worldwide, and 'Find Mii' in the US.
Streetpass Quest (Find Mii in the US) is an adventure game developed by Nintendo that comes pre-installed on every 3DS as part of the StreetPass Plaza.
You are on a quest to save your own personal Mii, who has been taken prisoner, and along the way, you can earn various hats that you can choose to put on your own personal Mii.
Every Mii you meet can be used as a hero in the game, or you can also hire mercenary heroes for Play Coins.