As unlikely as this combination sounds, it's made the Las Vegas quartet one of the hottest groups on the touring circuit -- after a high-profile slot at Lollapalooza last summer, it's returning to Chicago to headline the UIC Pavilion Wednesday -- and it has fueled sales of more than a million copies of "A Fever You Can't Sweat Out" since its release in September 2005.

"It was just really surreal," guitarist Ryan Ross says of the day the band learned that its debut went platinum. "When we were in the studio, none of us thought we would sell even 10,000 records. I don't know if there are any statistics that could prove this, but our record was probably one of the five cheapest records that have ever gone platinum! Nobody was really expecting anything from it, and it was our first try, so we were still trying to figure out what kind of band we wanted to be."

Nevertheless, a few influential fans heard the band's potential -- most notably Pete Wentz, the bassist and vocalist of Chicago's platinum-selling pop-punk heroes Fall Out Boy. He signed Panic! At the Disco to Decaydance, the imprint he started through Fueled by Ramen Records, based on the strength of a three-song demo.

"Pete was just really excited about what we were doing from the first time he heard us, and he invited us out on tour with them," Ross recalls. "That was a huge tour for us to do, and we were such a new band that I think we learned a lot just being forced to toughen up and play in front of that many people every night. It's also been really helpful for us to have Pete and the other guys to talk with about this business, because they've learned a lot of things the hard way."

Taking its name from a song by the Smiths, Ross formed Panic! in the Las Vegas suburb of Summerlin with his childhood friend, drummer Spencer Smith. The group, which is completed guitarist-vocalist Brendon Urie and bassist Jon Walker, started by playing covers by Blink-182, but it soon developed a much more complicated and nuanced sound.

"We've always listened to pretty much everything, from German Cabaret songs to thrash metal," Ross says. "There is a band that I was really into called East Side, and they did a bunch of old German Cabaret standards. From that, I started listening to more movie scores and stuff like the 'Chicago' soundtrack. Then, we got interested into doing something that was a bit techno-based, and we decided to start writing with things like drum machines. It all kind of evolved from there. The record really was kind of chronological in showing all these different things we were discovering.

"Making the album was a really stressful time for us, because we had never been in astudio before. It's a lot different from when you're at home and writing when you can -- you're in the studio for five weeks, 12 hours a day, seeing the same four people every day. We had never been around each other for that long, and then there was the added stress of trying to deliver something worth listening to."

The musicians weren't sure they'd succeeded, but after the album's release, the buzz began to build on the strength of tracks streaming from the band's MySpace site and the video for "I Write Sins Not Tragedies," which racked up more than 5 million views on YouTube. Befitting the circus-like music and the costume-crazed video, the group soon developed a theatrical and ever-expanding stage show; the current tour includes a ballerina, a juggler and "a girl who does this metal-grinding thing, where she has a saw blade she grinds against metal and sparks fly everywhere," Ross says. "We also wanted to have fire -- flame throwers and fire-breathers and stuff -- but it turns out you can't have any of that these days, since Great White."

Now, Panic! is gearing up for its encore: The plan calls for work on the next album to begin after this tour, with a likely release next spring. Once again, the band will be starting from scratch.

"I have never really read anything about another band that writes this way," Ross says. "It seems like most bands go into the studio and they have 20 or 25 songs they're working on, and they cut that down for what winds up on the album. We didn't cut any songs that we'd worked on: There are 13 on the first record, and that's how many we recorded. It takes us a lot of time to write songs, and we just end up using whatever we come up with! So we'll be starting all over again, but I think it will be really different this time, because now we know what we can accomplish."