INDIANAPOLIS—Without question Lucas Oil Stadium's retractable roof, which can close and open in 10 minutes, and its largest movable window wall in the world, open Sunday night, made it an architectural work to behold.

But for all the innovations inside the $720 million structure, the Chicago Bears always will remember it as the place they finally found the magical switch for 2008 they couldn't find for six long, frustrating weeks of preseason.

Consider it flipped.

The defense played with a purpose and passion that had been only a summer rumor and, thanks to silky rookie Matt Forte, the offense finally provided results in the running game to match Lovie Smith's rhetoric in a 29-13 upset of the Indianapolis Colts.

"I'm just glad we did turn it on because it really counts now," cornerback Charles Tillman said. "Nobody gave us a chance. We probably had three people who picked us to win this game. I'm sure all the people who bet on us in Vegas made a lot of money."

Which team playing in the Indiana capital Sunday night was a Super Bowl contender and which one was considered an underachiever on its way to a sub-.500 season?

In the stunning victory, the Bears made one of the louder statements of the opening weekend of the NFL season. Using fourth-down stops and fumble returns for touchdowns as exclamation points and key runs by Forte behind a resurgent offensive line as italic letters, it said, "We're back!"

Who knew?

The Bears insist they did and scolded naysayers for writing them off before the regular-season kickoff. On the joyful trip into the locker room, one player shouted, "Preseason don't count!" Another asked mockingly, "What are they going to write about now?"

Here's what: You know that bus the Bears always talk about getting off running? Well, it ran over the Colts on both sides of the ball.

"How does it happen? Get on the field," said Lance Briggs, who had a 21-yard TD return of a fumble stripped by Tillman. "We've been doing this for a long time. This group right here, no, we're not at all surprised."

The worst defense in the NFL during exhibition season didn't resemble the unit the Colts remembered from Super Bowl XLI. It looked better. Nastier. More focused and opportunistic. The sense of urgency that had been missing since players reported to Bourbonnais surfaced on the first series.

They mixed in a variety of blitzes to make Peyton Manning take as much time as the Bears hoped to develop a rhythm. Holes closed quickly, blocks were shed cleanly and suddenly a defense bad enough against the run in exhibitions to have to defend the validity of the Cover-2 was stopping the run. The Colts gained only 53 rushing yards.

When Adewale Ogunleye tackled Joseph Addai in the end zone for a safety in the second quarter, the defense had earned the benefit of the doubt it had lost.

"Even our die-hard fans were looking at it like, 'Maybe they can play well,' " said Ogunleye, who dominated. "Well, I knew what kind of team we had."

If not for a rare mistake by Devin Hester on special teams, the Colts probably wouldn't have scored a touchdown.

To start the second half, Adam Vinatieri booted one directly to Hester 7 yards deep in the end zone. Hester caught the ball and appeared ready to take a touchback when at the last second he decided to run it out.

It was the Bears' only dumb decision. The Colts swarmed Hester at the 3, and all of a sudden Chicago was pinned deep in its own territory, the crowd was back in the game and the Colts had momentum. On the next series, on cue, Manning steered the Colts into the end zone for the first time, culminating with a 6-yard TD pass to Reggie Wayne.

It may get lost in all the other highlights, but credit efficient Kyle Orton and the offense for being the buzz kill when the Bears got the ball back with the place rocking. The Bears didn't score on that drive but ran 12 plays and ate 7:12 off the clock before punting it back to the Colts.

"That was big," Orton agreed.

Having a bona fide NFL running back, which the Bears have lacked since trading Thomas Jones, makes such big drives possible.

Forte made history with 100 yards in the first half and 123 overall, and he will have few runs any more important than his 50-yarder for a TD with 4:59 left in the first quarter. More than any other play, that provided early confirmation to the Bears that they might be capable of pulling off the upset only they thought was possible.

"How about Matt Forte?" Lovie Smith gushed.

Jerry Angelo always was reluctant to use the word "special" to describe Anthony Thomas, the last rookie running back to make such an impact. So get out your football thesaurus to find another adjective for Forte in case Angelo thinks it's too early: exceptional, reliable, outstanding.

Finding words to describe what the Bears did Sunday night proved more challenging. You just had to see what maybe only the Bears believed was possible.