The game has a fantasy setting and your Mii is the king or queen of an unnamed realm. Everything seems to be going fine for him or her, but then they get kidnapped! You need the help of passing Miis to get you rescued, and the only way to collect these passing Miis is to literally walk by them on the street and collect the Miis via the StreetPass feature.
Miis all have different abilities based on the colour of their clothes and the more times you pass the same Mii, the stronger it gets (until it reaches its cap) and you use these Miis to progress through several different areas until you face the final boss and rescue the king or queen.
There's a very basic RPG gameplay style and you have to decide between having the Miis attack physically or with magic. There are different strategies to attacking the different kinds of enemies, so you have to make sure you're attacking in the right way. The king or queen will give you hints about the best ways to fight the enemies as you go along. The problem I found was that you are occasionally waiting for quite a while because you need a Mii with a certain kind of magic and you can end up stuck in the same place for ages because you don't happen to walk past a person whose Mii has that magic. The final boss is pretty cool though.
As a nice little bonus, occasionally when you defeat all of the monsters in a room you are rewarded with a treasure chest which will contain a hat for your Mii. These hats are sometimes even references to Nintendo franchises, such as Mario's cap and a hat which makes it look like Kirby is sitting on your head. You get to show off these hats to people you StreetPass too, as your Mii will be wearing whichever one you set when they fight for the king or queen in somebody else's game.
On the whole, it's nice but it's small. After I'd been few it a few times, I didn't really have any incentive to do so again. There's an achievement system which some people might enjoy, but I didn't care about it that much. For a game that's built into the system, it's pretty good, but compared to other, more substantial games, it isn't that great. I certainly appreciate that it was there however!
(Don't miss today's Finger Puppet Show!)