After spending the off-season signing big name free agents for their defense, the Minnesota Vikings began the season as the clear favorite in the NFC North. Peter King of Sports Illustrated even picked the Vikings to represent the NFC in the Super Bowl.

Instead, the Vikings began the season 1-4, a mess both on and off the field. The new additions to the defense couldn’t get on the same page. Quarterback Daunte Culpepper couldn’t find his receivers. All-Pro center Matt Birk declined to play hurt when the front office refused to guarantee his contract, and the offensive line crumbled in his absence.

After the fourth loss, a story leaked about an October 6 boat party on Lake Minnetonka which became a bacchanalia, complete with threatened sexual harassment lawsuits from the boat’s crew. The Vikings beat arch-rival Green Bay with a last-second field goal to win their next game, but a subsequent 38-13 loss to Carolina dropped them to 2-5. Even worse than the loss of the game was the loss of quarterback Daunte Culpepper to a season-ending knee injury, leaving the offense in the hands of 37-year-old retread Brad Johnson. Rock bottom wasn’t far off.

And then, having been left for dead, the Vikings proceeded to reel off five straight wins and vault back into the playoff race. Now 7-5, Minnesota is tied with Dallas and Atlanta in the race for the second wild card. If the Vikings can inch one game closer to division-leading Chicago (9-3), they will host the Bears in the final week to decide the division title.

What happened? Though he’s winning more games, most of the numbers suggest that Johnson isn’t playing any better than Culpepper. Both quarterbacks averaged 5.6 net yards per pass, and both converted on 30% of third downs for the season. Culpepper even had a higher completion percentage, 64% to Johnson’s 61%.

And yet, Football Outsiders’ DVOA (Defense-adjusted Value Over Average) system – which breaks down each play of the season and compares it to the NFL average based on situation and opponent – ranked Minnesota’s passing game 21st during the first eight weeks of the year. For the last five weeks, it ranks fifth.

So what’s the difference? Sacks and interceptions. Culpepper was picked off 12 times in 216 attempts before his injury – a ratio of one per 18 throws – with a sack every eight pass attempts. Johnson has thrown just two interceptions in 134 attempts – a ratio of one per 67 throws – with a sack every 10 pass attempts.

There’s good reason, however, to believe that Culpepper’s performance would have matched Johnson’s had he not been injured. Culpepper threw eight interceptions in the first two games of the season, against two of the league’s best pass defenses, Tampa Bay and Cincinnati. In his other four full games,he threw only four interceptions, including two against the Bears, whose defense could be regarded among the greatest in NFL history.

The bigger engine behind Minnesota’s turnaround is not pass offense but pass defense. Johnson was one of two Vikings to enter the starting lineup in Week 9. The other, to much greater effect if not fanfare, was rookie defensive end Erasmus James.

James is important not only for his excellent play, but because he personifies a change in Minnesota’s defensive philosophy. Defensive coordinator Ted Cottrell spent the first few weeks of the season experimenting with different players in the front seven, even trying the defense in a 3-4 alignment – a ludicrous idea considering that linebacker is the team’s weakest position.

But in Week 9, Cottrell pulled linebacker Raonall Smith and inserted James at defensive end, returning to a alignment. Suddenly, the Vikings not only had a pass rush, they had a pass rush that was strong with just the front four. This meant they could run fewer blitzes, which in turn meant the safeties could help the cornerbacks prevent deep passes.

The results have been miraculous. The Minnesota pass defense was ranked 25th in DVOA over the first eight weeks of the year. Since then, it has ranked third, with opponents dropping from 7.0 net yards per pass to 4.8.The Vikings had just nine sacks and six interceptions in the first seven games. In the last five, they have 14 sacks and 12 interceptions, including three in the red zone. Added pressure has also led opposing quarterbacks to throw to their running backs for shorter gains twice as often as they did before James entered the lineup.

Of course, it is no surprise when inserting a first-round pick into the lineup leads to improvement. But the other major lineup change for Minnesota is quite a surprise, because the defensive turnaround also coincides with an injury to Minnesota’s highest profile free agent signing of the off-season, cornerback Fred Smoot.

Smoot played much of the season with an injured shoulder, including the first game of the five-game winning streak. But he exacerbated the injury in that game when he broke his collarbone. For the last four weeks, he has been replaced by nickel back Brian Williams, the 2004 starter.

In the four weeks since Williams replaced Smoot alongside veteran Antoine Winfield, the completion percentage allowed on passes to opponents’ no. 1 receivers has dropped from 60% to 38%, and the average yardage on the completed passes has dropped from 15.8 yards to 13.1. In their most impressive game, Williams and Winfield limited the Giants’ Plaxico Burress to three catches on 12 passes and just 50 yards. It will be interesting to see what changes if Smoot returns from his injury soon, as expected.

If the Vikings can seamlessly integrate Smoot back into the defensive lineup, they could become just the 10th team in NFL history to reach the playoffs after beginning the season 1-4. Atlanta and Dallas each face tough opponents over the last four weeks, but Minnesota’s schedule is favorable: only one road game, against depleted Baltimore, and home games with mediocre St. Louis and disintegrating Pittsburgh. When Chicago visits on the last Sunday of the year, the Vikings could be playing for a spot in the postseason or even the division title, an idea that seemed impossible just one month ago.

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