The nation’s most electric returner looked up as a second-quarter Wisconsin punt came hurtling in his direction. Zachariah Branch had fielded so many kicks just like it. But as this one drew closer and a decision drew near, for a split-second the USC sophomore hesitated.

It was an uncharacteristic mistake at the start of an error-filled afternoon, a fitting moment to explain a mostly calamitous first half. The ball careened off Branch’s outstretched arms, slipped through another USC player’s hands and was eventually corralled by Wisconsin. And with it, any hope of a smooth, bounce-back spot for No. 13 USC in its Big Ten home opener seemed to slip away, fading into another ambling start.

What USC got instead was a confounding afternoon that even the Trojans and their coach — and anyone in attendance at a sold-out Coliseum, for that matter — would probably struggle to explain. It was filled with an equal mix of first-half frustration and second-half steely resolve. A slow, painful start defined by mistakes like Branch’s botched return gave way to a resounding, confident second half defined by the exact opposite.

Trojans quarterback Miller Moss gets a pass off before Badgers defensive lineman Curt Neal can get the sack Saturday.

(Gina Ferazzi / Los Angeles Times)

USC climbed out of a halftime hole to score four unanswered touchdowns, walking away with a 38-21 win over Wisconsin, its first in the Big Ten.

Bouncing back after last week’s back-breaking loss to Michigan required another masterful run of halftime adjustments from USC defensive coordinator D’Anton Lynn, whose defense allowed Wisconsin zero points and 82 total yards in the second half. It also took a gutsy comeback effort from quarterback Miller Moss, who turned around a sloppy start to lead three consecutive touchdown drives in the third and fourth quarters, running the last one in himself.

“After the way that last week finished,” USC coach Lincoln Riley said, “to have the first half go the way it did, then to come back and play well, says a lot about the character of the guys in this room. They’re going to battle. We’re not going anywhere. We don’t panic.”

That was especially true of Moss on Saturday. The redshirt junior didn’t look sharp to start against Wisconsin. He threw an ill-advised interception and handed away a back-breaking fumble in the red zone. As an offense, USC struggled to find its rhythm as receivers dropped critical passes and the offensive line missed key blocks, picking up where it left off last week.

Little was working. Which, Riley reminded, should have felt familiar.

USC receiver Duce Robinson makes a 32-yard reception against Wisconsin safety Austin Brown to set up the go-ahead touchdown.

(Gina Ferazzi / Los Angeles Times)

“It’s the second week in a row we’ve been in the locker room, and it’s like, ‘Alright, are we going to step up and go play the way we know we’re capable of?’” Riley said.

Moss, all along, kept his calm.

“It wasn’t like they were doing something that was world-beating and we couldn’t execute,” Moss said. “We just knew we had to play our brand of football, and we’d be fine.”

That brand was certainly on display in the second half. Even as pressure bore down and the hits piled up, Moss delivered big throw after big throw, hitting Ja’Kobi Lane in the corner for a touchdown on one drive, then finding Duce Robinson over the middle in the face of an all-out blitz on the next.

Moss finished with 308 yards and three passing touchdowns, plus another on the ground. He’d pay for that final score, though, spinning out of one tackle before colliding into another as he leaped into the end zone for a game-sealing score. He fell backward through the air and his helmet hit the turf on the play.

USC quarterback Miller Moss (7) scores on a fourth-down play against Wisconsin in the fourth quarter at the Coliseum on Saturday.

(Gina Ferazzi / Los Angeles Times)

“You go flying, you see some things sometimes,” Moss said with a smile.

The hit briefly sent him to the medical tent. But the quarterback brushed it off later, even as teammates marveled at his toughness.

“He’s a dawg,” Lane said.

“You see him get smacked, get back up and throw touchdowns,” linebacker Mason Cobb said. “Man, we’re a team that’s going to keep swinging. That’s our identity, for real.”

That certainly wasn’t the first thing anyone would say about USC a year ago. No one, especially, would have confused USC’s defense for a resilient group.

And at the start Saturday, that unit seemed to slide briefly back into bad habits. Wisconsin opened with an explosive passing score.

But the uneven effort was ironed out at halftime, as it has been under Lynn. The defense refocused.

“We just woke up, to be honest,” defensive end Jamil Muhammad said.

And in the second half, USC shut down Wisconsin. Just one of the Badgers’ drives after halftime spanned more than four plays.

The difference from that point, on all fronts, was undeniable. Moss found his stride. The defense found its mettle. The early mistakes disappeared.

Where Branch muffed a punt in the first half, Wisconsin muffed one in the second, handing USC a much-needed third-quarter break.

USC scored soon after. Wisconsin never did again.

It was a stunning turn from the Trojans, given all that had gone awry early. Asked what had happened in that halftime locker room to spur such change, Riley smirked.

“Just a really, really good speech,” he said.