During last Saturday's 19-10 win at Duke, Northwestern's coaches sent in a defensive play. The call, however, didn't fit the personnel on the field.

Said personnel noticed the mistake.

"The D-line comes off and they're ripping my rear end," head coach Pat Fitzgerald said. "[They say], ‘You can't call that. What are you doing?'

"That just shows you the depth of football IQ this defense has right now."

That IQ, honed through experience, is a major reason why the No. 17 Wildcats have fielded the stingiest defense in the country through three weeks. No, seriously.

Northwestern has allowed only 16 points in three games, the fewest in the FBS, and Duke scored the lone touchdown against that defense this season. Fitzgerald's team has yielded just 235 yards per game (sixth nationally), 3.81 yards per play (fifth) and leads the FBS in third-down conversion percentage allowed (14 percent).

They've done it against a schedule that includes two solid Power 5 opponents, too. A season-opening 16-6 win over Stanford looks even more impressive after the Cardinal scored 41 points in a win at USC last weekend.

This is the same bunch of players, for the most part, that finished ninth in the Big Ten in total defense a year ago. So what's behind the defensive surge that's fueled the Wildcats' 3-0 start?

Defensive coordinator Mike Hankwitz said the No. 1 reason for the improvement is senior leadership. He can count on seniors at every level of the unit, from defensive linemen Dean Lowry, C.J. Robbins and Deonte Gibson, to linebacker Drew Smith, to defensive backs Nick VanHoose and Traveon Henry.

"It's a strong class," Hankwitz said. "They take it upon themselves to set the standard for young players and really for everybody."

The experience edge goes beyond just the upperclassmen, however. Northwestern was racked by injuries last year. The upside was that several young players got immediate playing time, like linebacker Anthony Walker, safety Godwin Igwebuike and cornerback Marcus McShepard. That not only advanced their development, but it brought newfound depth to the roster once other players returned healthy.

"You can go hard each play as a ‘1,' and the guys behind you can play just as well," said Lowry, who played only 60 percent of the snaps against Duke. "Overall, we have better depth than previous years."

Efforts by Fitzgerald and his staff to add more speed on defense have also begun to pay off. Hankwitz called this the fastest group he's coached at Northwestern, which is saying something since he's been in his current role since 2008.

That speed is typified by Walker, who had 19 tackles vs. Duke to earn his second Big Ten defensive player of the week award in three games. The sophomore from Miami has been making plays from sideline to sideline.

"He's just fun to watch," Lowry said. "He triggers on the ball so fast. Having a guy like that play behind you gives you all confidence in the world. Even if you make a mistake, he's an eraser."

More speed is great, but the Wildcats wanted to make sure this offseason that they could hold up against the run-heavy Big Ten teams on their schedule. Hankwitz credits the strength staff for adding weight to several players' frames. Lowry said he's put on 15-20 pounds every year he's been in Evanston, and he was a nightmare for Duke to control last week as he had an interception, a tipped pass he almost took in for a touchdown and several quarterback pressures. Fitzgerald named him, not Walker, the team's defensive standout for the game, saying the defensive end was "just dominant. All over the place."

"I'm definitely controlling blocks better and setting up blockers, which is key," Lowry said. "I didn't go inside as well early in my career. But now I can bull rush a little better and be physical at the point of attack."

That Northwestern held a physical Stanford offense to just 240 total yards and 85 on the ground in the opener is a promising sign for success moving forward, especially when the Big Ten season opens after this week's game vs. Ball State.

Hankwitz said he sensed something good might be brewing this offseason because of how well-coached and how determined the players were. Much of that stemmed from back-to-back 5-7 seasons and some heartbreaking losses the past two years.

"I think they just realized how close they were," he said. "And they made it their mission to get there."

The Wildcats are back in the Top 20 for the first time since October 2013, powered by a defense that has compensated for a still-developing offense. When you only allow one touchdown every three games, that can carry you a long way.

"It's something special right now," Lowry said. "We've just got to keep it going."