The twin brothers, Tatsuya and Kazuya Uesugi and their neighbor Minami Asakura get sucked into a alternate dimension while chasing after the twins dog, Punch. Now they have to find a way to return back to their normal home world.

City Adventure Touch: Mystery of Triangle is a beat-'em-up. Players take control of the twin brothers (players can switch back and forth between each brother in the one player game) and explore the maze-like city, trying to locate and collect ten dogs. Several enemies roam the streets and the twins can throw punches or baseballs at them in order to defend themselves. Their are also shops which sell items, for example food shops which sell restorative health items.