To understand exactly why this Indiana team is different—besides the top-10 ranking and undefeated record, of course—it’s helpful to look at last year’s game against Michigan State and compare it to the one on Saturday, which was won easily by the Hoosiers, 24-0.

Last year, Michael Penix Jr. played as well as he possibly could have, completing 20 passes in a row, which was the second-most in Big Ten history. And it still wasn’t enough, as Indiana’s defense faltered in a 40-31 defeat. On Saturday, Penix was far from perfect—in fact, he didn’t play that well at all—and yet the Hoosiers cruised to a win.

Who would’ve thought that as we near Thanksgiving, Indiana (4-0) would have as many wins as Penn State, LSU and Tennessee—combined?

And granted, most expected Indiana to have a big season after last year’s breakthrough. But the prevailing opinion was that it would be because of the wealth of talent returning offensively, led by skill players Penix, Whop Philyor and Stevie Scott. Instead, it’s been Indiana’s defense that has emerged as the driving force behind the 4-0 start. On Saturday, the Hoosiers intercepted 3 passes—their fifth straight game with multiple interceptions, dating to last season—on their way to notching their first shutout in Big Ten play since 2000.

Indiana’s defense is the reason that the Hoosiers (4-0) have a chance at No. 3 Ohio State (3-0) next Saturday in what will be the most important game in program history since, well, maybe ever. The winner will, in all likelihood, go to the Big Ten Championship Game. The team that beats Ohio State will need to play a complete game, no doubt. That doesn’t mean shutting the Buckeyes out, but it does mean holding them around, say, 30-35 points. That would at least give a team with offensive talent, like Indiana, a chance. A Justin Fields-led Buckeyes team has been held under 30 points twice in 17 games.

Indiana has been building this defense up under Allen, and after 3 years of calling the defense, he handed it over to 33-year-old Kane Wommack last season. Indiana allowed 24.4 points per game last season, resulting in the Hoosiers being ranked in the top 50 in scoring defense for the first time since 1993. But subtract a few cupcakes in the non-conference, and Indiana actually allowed 29.7 points per game. That’s the best way to examine the improvement since this season is a conference-only schedule.

After shutting out Michigan State, Indiana is allowing 19.3 points per game, which could bump the Hoosiers all the way into the top 20 nationally. It’s certainly going to get more challenging, as Ohio State, Maryland, Wisconsin and Purdue await—all of which have at least shown flashes of a good offense. But as the Hoosiers have shown, they are more than up for the challenge. The 191 yards Indiana allowed was the third-fewest it has allowed in a Big Ten game since 2000.

Indiana entered Saturday fifth nationally in turnover margin—the only team in the top 14 that has played fewer than 5 games. After the 3 interceptions and a fumble recovery, the Hoosiers will probably stay in the top 5.

It’s not a huge surprise that there are Hoosiers having good seasons. Pro Football Focus named sophomore cornerback Tiawan Mullen as 1 of the B1G’s top returning players regardless of position, and he showed why on Saturday with 2 interceptions. His first one turned the momentum a bit and gave Indiana a shot of life after Penix threw an ugly first-quarter interception. Micah McFadden, Indiana’s leading tackler and a team captain, is a proven B1G linebacker. It sounds cliché, but Indiana’s defense has come together and is greater than the sum of its parts. They’ve shown it with their play and with the way they treat their teammates, like the way about 15 team of them rushed the field to wish Thomas Allen well when he was carted off the field Saturday. You can’t fake that sort of enthusiasm.

Because Indiana’s defense has been so good, it’s overshadowed a bit of a disappointing start from Penix, who entered Saturday averaging just 6.7 yards per attempt (10th in B1G). While Penix threw for 320 yards and 2 TDs, he had several head-scratching throws—2 of which resulted in interceptions and a few others that could have been picked. Greg McElroy, a former Alabama QB who was calling the game for ESPN, pointed out that Penix’s footwork needs improvement.

To win, Indiana obviously needs a dynamite game from Penix and guys like Ty Fryfogle, who went absolutely nuclear with 11 catches for 200 yards and 2 TDs on Saturday (and has 342 yards over the last 2 games). You won’t beat Justin Fields without an explosive offense.

But they also need this defense to continue this impressive run it is on. Indiana has allowed at least 49 points in 3 straight games against the Buckeyes and hasn’t held them under 30 since 2001—a span of 17 games.

Even though Fields has been near unstoppable the last 2 seasons, Indiana has its best shot to stick with Ohio State, thanks to a ball-hawking, hard-nosed defense.