LOS ANGELES — If ever a college football game was set up to have a perfect Hollywood ending for the home team, it was Saturday night’s scrum at the famed Los Angeles Memorial Coliseum between the nationally ranked BYU Cougars and coach-less, tradition-laden USC Trojans.

You know, a freshman quarterback from Utah making his second start throws a game-winning touchdown pass to beat one of his hometown teams?

Well, the Cougars (10-2) had other ideas.

“We knew we were going to get USC’s best shot, and we did. It was looking pretty bad, but once we regained our composure, I thought the guys rallied back and we felt really positive that we were going to get this win.” — BYU football coach Kalani Sitake.

A BYU defense that had given up 453 yards finally rose up when it had to, holding the Trojans to five yards when they needed six, and the No. 13 Cougars rode off with a 35-31 win to complete the 5-0 sweep of Pac-12 teams, adding USC to the likes of Utah, Arizona, Arizona State and Washington State.

“We ended up punching them in the mouth,” said BYU running back Tyler Allgeier, who rushed 21 times for 111 yards and two touchdowns, but wasn’t on the field when the Cougars punched in the game-winning touchdown with 3:57 remaining.

Redshirt freshman Jackson McChesney did the honors, plowing to the end zone from 7 yards out to give BYU what would turn out to be its first come-from-behind win since it downed Houston midway through the 2020 season after trailing at halftime.

This time, the Trojans led much later than that.

“We knew we were going to get USC’s best shot, and we did,” said BYU coach Kalani Sitake. “…It was looking pretty bad, but once we regained our composure, I thought the guys rallied back and we felt really positive that we were going to get this win.”

Unlike last year, when Zach Wilson and a powerful offense carried the Cougars, their banged-up defenders have been the unsung heroes in 2021, and that proved true again on a picture-perfect late-autumn night in Tinseltown.

Trailing 35-31 with a fourth-and-6 situation at the BYU 15, Trojans QB Jaxson Dart, the former Roy and Corner Canyon High star and Deseret News’ Mr. Football last year, threw a slant pass to Gary Bryant Jr.

However, BYU’s Kaleb Hayes stopped Bryant a yard short of the line to gain, touching off a wild celebration on the Cougars’ sideline.

The Cougars had flipped the script.

“There was no doubt in my mind that we were going to get that fourth-down stop,” said BYU linebacker Max Tooley, who tied Ben Bywater for game-high tackling honors with 13.

“Football is a crazy game. It can flip in the blink of an eye.”

It did on this night. The Trojans seemed to be in control with eight minutes remaining after Calen Bullock picked off Jaren Hall in the end zone on second down, preserving a 31-28 lead.

A holding penalty helped the Cougars force a three-and-out, however, and Hall and company got the ball back at their own 38 with 6:19 left.

A zany play in which Allgeier fumbled, but tight end Dallin Holker scooped the ball up and advanced it down the field, making the play 30 yards total, moving the ball to the USC 25.

About that time, McChesney spelled Allgeier and had three carries for 17 yards and the go-ahead TD.

“Tyler was just spent,” Sitake said.

Allgeier said he still had something left in the tank, but was happy to see McChesney get the ball.

Fresh legs might have been the difference, as McChesney broke several tackles on his way to the end zone.

“It was great. … It was like the cherry on the cupcake,” Allgeier said, mixing his metaphors a little bit. 

For the Cougars, now 10-2, it probably wasn’t enough to move them up in the College Football Playoff rankings, but it was satisfying nevertheless. 

What else did they learn?

That sometimes it is better to be lucky than good. For instance, on BYU’s first possession of the second half, Hall was intercepted by Chris Steele, but Drake Jackson was flagged for roughing the passer.

BYU finishes season a perfect 5-0 vs. the Pac-12

Five plays later, Hall threw a 41-yard touchdown pass to Keanu Hill on a ball that was misjudged by the USC defensive back.

The Trojans didn’t turn the ball over, but they came away with just three touchdowns in seven trips to the red zone.

“They gave us a fight and luckily we were ready for it,” said Hall, who completed 20 of 32 passes for 276 yards and had a quarterback rating of 143.1.

Aside from the final throw that didn’t get to the sticks, Dart outplayed his fellow Utah prep product, completing 23 of 35 passes for 248 yards and a TD.

“That’s what this team is all about, literally,” Allgeier said. “We have that trust in each other that we will get it done.”

Dart played masterfully in leading the Trojans on the 91-yard drive to start the fourth quarter that ended with a 10-yard touchdown toss to Bryant.

Several times he escaped sacks, eluding the grasp of BYU’s Gabe Summers on one sensational Houdini act, and also used that familiar running style to get chunks of yardage.

Bryant’s TD catch put the Cougars in a position they haven’t been in often — trailing in the fourth quarter — in the past two years when they’ve gone 21-3 and defeated six Power Five foes.

“We rallied back,” Tooley said. “We trusted the process.”

3 takeaways from No. 13 BYU’s back-and-forth win over USC

Part of that process was watching their head coach get an unsportsmanlike conduct penalty that cost his team 15 yards.

The Trojans used that and Vavae Malepeai’s 35-yard run to get into field goal position, and Parker Lewis’ 37-yard field goal cut BYU’s lead to 28-23.

“I lost my cool,” Sitake said. “…It wasn’t my best moment. … But I think every coach wants to fight for his guys. I guess on that one I stepped out onto the field too far.”

At about that time, it looked like the Cougars’ mounting injuries on defense were going to catch up with them.

They were also shorthanded on offense, with receiver Gunner Romney not playing after tweaking something in warmups and tight end Isaac Rex suffering a serious ankle injury while trying to haul in a touchdown pass.

“We were down a bunch of guys on defense,” Sitake said. “We had to rely on our depth.”

In the first half, the Cougars seemed on the verge of taking control, only to see Hall get picked off for the first time in 164 passing attempts.

Hall’s pass was tipped by Isaac Taylor-Stuart and intercepted by Kana’i Mauga, giving the Trojans the ball at BYU’s 30.

USC took over with 1:05 left and drove into field goal position; Lewis’ 33-yarder cut the Cougars’ lead to 21-13 at halftime.

The Trojans took a 3-0 lead. Gary Bryant Jr. took the opening kickoff 61 yards, and USC struck first with a 26-yarder by Lewis, the Cougars’ defense stiffening after USC moved the ball to their 6 yard line.

BYU marched 75 yards for a TD on its first possession, with Hall throwing a 28-yard strike to Puka Nacua after Holker’s 18-yard reception.

Allgeier’s 9-yard TD run gave BYU a 14-3 lead with 11:34 left in the second quarter. It was the 10th straight game in which Allgeier has scored a touchdown, tying Mike Chronister’s school record.

Allgeier’s 5-yard run gave BYU a 21-10 lead with 5:45 left in the half and the Cougars got the ball back after a rare USC three-and-out, but the interception reversed the momentum.

The Cougars had 279 yards in the first half; Hall was 14 of 22 for 167 yards and a touchdown, with a passer rating of 133.3, despite the pick.

As for the Cougars improving to 5-0 against the Pac-12 and some fans chanting “Pac-12 champs” after the final whistle, Sitake wanted nothing to do with that kind of talk.

“We are just happy to play the games (against the Pac-12 teams),” he said. “We are not going to talk down on anybody.”

That just wan’t in the script, either.