If you haven't heard of alternate-reality games, it probably means you're living in North America, where their popularity, though rising, lives on the fringes. But a United Kingdom-based maker of a leading alternate-reality game wants to expand that market scope.

　　More popular in Europe, Perplex City--an alternate-reality game that combines real-world actions and traditional card gaming--debuted in the United States in February with a New York event, but now the game's maker is hauling the card series to the West Coast. Mind Candy, headquartered in London, is staging a Perplex City-themed scavenger hunt Saturday in San Francisco, starting at 10:30 a.m. at the Palace of Fine Arts Theatre. Up to 500 people are eligible to play and another 500 can watch by registering at the Perplex City Web site.

　　Gamers may already be familiar with marketing-based alternate-reality games such as Bungie Studios' viral-marketing campaign for Halo 2, ILoveBees. The game's players collaborate in real life, teaming up for such tasks as rooting out storyline clues in mass, real-world scavenger hunts that blend the line between the real world and the game world. Unlike ad campaigns such as ILoveBees, Perplex City was created first and foremost as a game, Mind Candy spokesman Rob Fleischer told GameSpot.

　　"You can see games like Half-Life moving in this direction with their release of game episodes," he said. "What Perplex City is doing is taking that to its natural extension and making the game truly live, so it grows organically in both gameplay and story."

　　For more on Perplex City, visit its Web site.