The Nuggets are a puzzle, a bite-your-lip, shake-your-head, look-at-the-ceiling puzzle.

They are now 24-12, which is very good, and 17-1 against teams with losing records, which is crazy good. By the record, they are as good as they've been this far into a season since they joined the NBA in 1976.

Nevertheless, their first half Monday night against the cellar- dwelling Indiana Pacers in a sparsely populated Pepsi Center was a case study of the puzzle.

For the first eight minutes, with their starters on the floor, the Nuggets were terrific. When George Karl made his first substitution - J.R. Smith for Dahntay Jones with 4:20 remaining in the first quarter - they led 31-14.

They were sharing the ball and playing active defense. Carmelo Anthony had five assists, on his way to nine for the game.

Pacers star Danny Granger, fifth in the NBA in scoring, had two points with Jones checking him.

Once Karl went to his reserves, the chemistry disappeared. As soon as Jones sat down, Granger heated up. After scoring 40 in the first quarter, the Nuggets surrendered an inexcusable 42 in the second, the most they've given up in a quarter all season. Up 20 late in the first quarter, their lead was four at the half.

"We got off to an aggressive start offensively and defensively and we just had it going too easy for the second (game) in a row," Chauncey Billups said afterward. "We started letting them kind of have their way because we were scoring so easily. What happened was it bit us in the behind."

When I mentioned the turn seemed to come when they went to the bench, Billups did what leaders do, which is stress group accountability.

"You can't pinpoint anybody," he said. "It's all of us. You see one guy doing the wrong thing, it's on all of us to step up and hold them accountable. It's not on one player or one group or one unit. It's on all of us."

In the space of one half, the Nuggets were as good and as bad as they can be. And yet, by the end, they were cruising comfortably to their fourth win in a row.

Are they for real? By winning percentage, only the Lakers and Spurs are better in the West. Have the Nuggets finally arrived as a genuine contender or are they still mining fool's gold?

"I was as impressed with this group as I've been with any team in the league in scouting them," Pacers coach Jim O'Brien said. "Certainly, I think you have to give the edge to the Lakers, but the difference between (the Nuggets) in the past when I scouted them and them with Chauncey Billups (is) the other guys understanding that defense is the way that you get into contention to win a championship.

"I think they're one of the toughest teams in the league from the standpoint of how you defend them because they have the consummate point guard in Billups and they've got a tremendous amount of weapons.

"Defensively, you just look at their statistics - they're in the top 10 in field-goal defense, they are strong and athletic and they have good quickness to be able to keep people in front of them. I think they're absolutely for real. And I think anybody who thinks differently will find out that I'm right at the end of this season."

Although Anthony, Billups and Nene get most of the publicity, Kenyon Martin and Jones hold the keys to the Nuggets' aggressive team defense. But the vulnerability of the bench raises a legitimate question about what happens in the event of a significant injury to the starting lineup, starting with the "probable nondisplaced fracture" Anthony sustained to his right hand in the third quarter.

As much as they would miss their star if he has to miss an extended period, the nature of their bench gives them a better chance to withstand an injury to Anthony than to either of their big men.

"We've got a really good team and we've got a chance to get to the playoffs and advance," Billups said. "And from there, anything can happen. But we don't want to get too far ahead of ourselves. We need to get better at every juncture of the season, and right now, our next level is, when we get teams down, learn how to keep them down. We've got a lot of young guys that really haven't probably been in situations where they get this many teams down early in the games. So it's a process."

It is as promising a group as the Nuggets have had in a long time. They have a chance to earn home-court advantage and a better first-round playoff matchup than this franchise has seen in the last five years of one-and-dones.

But the fact remains that they are 7-11 against teams with winning records. That's just another part of the puzzle.