The Indianapolis Colts offense is as potent as always. The Denver Broncos have allowed just 7.3 points a game, the lowest in the league. The winner of this battle of strengths will move into the driver’s seat in the AFC playoff race.

INDIANAPOLIS COLTS (6-0) AT DENVER BRONCOS (5-1)
(Sunday, 4:15 p.m.)

The Broncos are 1–4 against Indianapolis since 2001; watch this game, and you’ll hear a lot about the Colts eliminating the Broncos from the playoffs in both 2003 and 2004. When the Colts exposed the Denver secondary by knocking them out of the 2003 playoffs with a 41–10 victory, the Broncos traded star running back Clinton Portis for Pro Bowl cornerback Champ Bailey. The next year, the Colts beat Denver in the playoffs again, 49–24, simply passing to whichever receivers were not covered by Bailey. Again, Denver tried to fill holes exposed by the Colts, drafting three cornerbacks in the second and third rounds of the 2005 draft.

As a result, the Broncos may now have the deepest secondary in the league. Combine that with a strong set of linebackers, and you have one of the best defenses in the league. Denver has only allowed two offensive touchdowns in six games this year, both late in games where the Broncos had a comfortable lead.

But there are reasons to believe that Denver’s defense is not quite the juggernaut it seems to be. The Broncos have been stingy with points, but not necessarily with yardage. Opponents average 23.7 yards a drive, which ranks the Broncos sixth in the league. The defense has enjoyed an easy schedule; the last three opponents were Baltimore, Oakland, and Cleveland, three of the five worst offenses in the league in yards per play. It’s also harder to score when you don’t get the ball very often, and Denver has faced just 65 offensive drives this year, the third fewest in the league.

Football Outsiders’ Defense-adjusted Value Over Average ratings (DVOA) — which break down each play of the season and compare it to the NFL average based on situation and opponent — rank Denver as the top red zone defense in the league. In fact, DVOA ranks Denver as the top red zone defense since 1997, the first year for which we have play-by-play breakdowns. Never in that time has there been a team with a larger gap between overall defense and redzone defense. Denver’s red-zone defense is certainly going to be strong all year, but this current level of play is simply unsustainable.

Ironically, the Colts may not present the best test of Denver’s historic red zone supremacy. On the other 80 yards of the field, the Colts have a DVOA twice as high as any other offense in the league. But this year, they have been just an average offense once they pass the 20-yard line. Peyton Manning’s playfakes become much less dangerous without the threat of the deep pass, and the Colts’ running game tends to stall out as it gets closer to the goal line. Part of the problem is too many carries for veteran Dominic Rhodes (3.1 yards a carry) and not enough for rookie Joseph Addai (5.1 yards a carry).

The Colts may not be able to match Denver’s strength in the red zone, but they will match Denver’s other strength: third downs. The Broncos are the best defense in the league on third downs, but the Colts are the best offense in the league on third downs, and by a huge margin. The Colts have converted 57% of third-down opportunities, with no other offense higher than 47%. Denver, meanwhile, has allowed conversions on just 28% of third downs.

The problem for Denver is less the Colts offense, and more their own. The Broncos have yet to win a game by more than 10 points because their offense has been nearly as anemic as their defense has been powerful. The running game is fine, with Tatum Bell averaging 4.7 yards a carry and finally establishing himself as the starter after sharing time for two seasons. Quarterback Jake Plummer, on the other hand, has been awful.

When Plummer started the season slowly, it seemed like a normal slump, the kind every athlete endures. But six weeks into the season, nothing has turned around. Plummer has a completion percentage of just 52%, far below his 61% from last season. In fact, it is the lowest completion percentage of his career, lower than even 1999 when he had one of the 10 worst quarterback seasons of all time. His average of 5.8 yards per attempt is more than a yard below any season since he came to Denver in 2003, and he’s already thrown as many interceptions as he did last year (seven).

Plummer’s problems are exacerbated by a struggling group of receivers. The only one having a good season is Javon Walker, acquired in the off-season from Green Bay. In particular, veteran Rod Smith seems to finally be slowing down at the age of 36. Walker has been thrown just five more passes than Smith, but he has seven more catches and 290 more yards. Denver has never relied on more than two wide receivers, and this year is no different; David Kircus is third on the team with just 62 yards. Tight ends, usually a big weapon in Denver, are also a problem. Veteran Stephen Alexander hasn’t been good for years, and rookie Tony Scheffler has caught just two of the 13 passes thrown to him.

The Colts’ defense could be just what the doctor ordered for the struggling Denver offense. Last year’s defensive improvement in Indianapolis has disappeared, as the Colts rank 22nd in pass defense and 29th in run defense according to DVOA. The center of that run defense got a lot stronger last week when the Colts dealt a second-round pick to Tampa Bay for nose tackle Anthony “Booger” McFarland, but he was forced into the starting lineup immediately when a car accident knocked out tackle Montae Reagor. The Colts are also banged up in the secondary, where safety Bob Sanders has missed four games after arthroscopic surgery on his right knee, and his backup, Mike Doss, was just placed on injured reserve.

The Broncos don’t have as many injuries as the Colts, but they have one very big injury that stands out. Left tackle Matt Lepsis is the heart of the Denver offensive line, starting every game but one since 1999. Last week, a torn ACL ended his season, and the possible replacements are not promising. Lining up against Colts right end Dwight Freeney, one of the top pass-rushers in the game, will be rookie Erik Pears, current right guard Cooper Carlisle, or veteran Adam Meadows, who hasn’t played a game since 2003.

All these storylines add up to two evenly matched teams and the best game of the NFL season so far. The Broncos will probably set season highs for points scored and points allowed on Sunday, but it’s impossible to guess which of those numbers will end up higher.

Mr. Schatz is the editor in chief of FootballOutsiders.com.