SEATTLE Glance at a recent computer game sales chart and you'll see that nestled between the typical combat and science fiction titles lies something a little different.
That would be "Mystery Case Files: Ravenhearst", which was the third-best-selling PC title in the United States for the week ending in the annual Black Friday shopping splurge.
Gamers who got lost in super-realistic fare like "Call of Duty 4" and "Crysis" may scratch their heads, but a growing number of casual players will nod knowingly at the mention of the leading example of the "hidden object" genre.
"Hidden object" titles are virtual scavenger hunts in which players hunt for items against minutely detailed backdrops, often racing to beat a clock or reveal clues to an overarching mystery.
Seattle's Big Fish Games essentially defined the genre with the "Mystery Case Files" series two years ago. The company estimates that 100 million people have at least sampled trial versions of the games since then.
The fourth installment, "Madame Fate", launched last month for download on Big Fish's Web site, sold 100,000 copies in six weeks -- big numbers for a casual game.
Unlike a hard-core game franchise like "Call of Duty" where each sequel overshadows its predecessors, the debut of the new "Mystery Case Files" title rekindled interest in past games. Hence the strong retail sales of "Ravenhearst", the third title in the series.
"When it hits retail during the holidays, people recognize the brand and buy it," Big Fish founder Paul Thelen said in an interview.
"Our best guess -- we don't have actual data -- is that it's people who have played or bought 'Case Files' online and now, with the holiday coming, they want to give that to friends or relatives," Thelen said.
Another difference between "Mystery Case Files" and a big title for a gaming console like Sony's PlayStation 3 is that a big console game usually makes a splashy debut and then sees sales taper off.
Big Fish, however, gets more mileage out of "Mystery Case Files" by first posting it for download and then offering it more broadly in shops and on other Web sites several months later, which leads to a second spike in sales.
The first game took about six months to program, but Big Fish, not wanting to rush what it sensed could be a hit, spent more than twice as long before that polishing the design.
"We initially came up with concepts that didn't work well. At first it was a kids' game, and that didn't work well with audiences," Thelen said.
Big Fish has cranked out four "Mystery Case Files" games in two years, but that pace is set to slow a bit as developers get more ambitious.
"We would like to get one out every year because it is episodic," said Paul Handelman, head of business development for Big Fish. "We are really trying to think of ways to bring the whole brand together."
Like all success stories, "Mystery Case Files" has spawned ranks of imitators, and even Big Fish has rolled out other hidden object games, such as its "Hidden Expedition" series, and a new one featuring 3D, 360-degree scenes in London.
"Mystery Case files" mainly appeal to women aged 35 to 50, but a mobile version launching in partnership with Glu Mobile next year should appeal to younger players who are more likely to play games on their phones.
"We're pushing the limits in terms of what has ever been done on mobile, in terms of the graphics quality, use of the networks, and game design," Handelman said.