LOS ANGELES — The sun went down, and USC’s worst nightmares came to life.

The defense that looked dominant last week was coming apart at the seams, shredded by a first-time starter at quarterback in Stanford’s Tanner McKee. The problematic penalties returned, the discipline diminished, the force field inside the red zone remained. Even the kicker was kicked out.

In its first taste of #Pac12AfterDark, USC fell asleep at the wheel in a 42-28 thrashing by Stanford.

Where its beleaguered coach stands after USC’s most disastrous defeat in years remains to be seen. But as Clay Helton and his team left the field Saturday night, a chorus of boos rained down. After a nightmarish night, they could only hang their heads.

The Trojans’ long night started with kicker Parker Lewis ejected for targeting. Things got only worse.

After holding San Jose State to 67 yards on the ground, USC gave up 87 on a single, first-quarter rush, as Stanford’s Nathaniel Peat found a crease, slipped a shoestring tackle and sprinted to pay dirt. The big play more than doubled the Cardinal’s rushing total from the previous week, when they didn’t manage to score until late in the fourth quarter.

This time it was USC slogging its way through possessions, its offense unable to conjure more than a few yards. Kedon Slovis couldn’t find a rhythm, managing just 223 yards passing.

It would take a 15-play, 95-yard drive for USC to punch in its first touchdown, and even then, it wasn’t easy, as Keaontay Ingram pushed his way into the end zone for a hard-earned two-yard score.

Nothing was easy, as USC wandered up and down the field, unable to find the end zone again until six minutes remained and the game was well in hand. Twice more, USC made it inside the 20, only to be turned away.

Its defense, fresh off a strong debut, seemed determined to make matters worse, extending drive after drive with ill-timed penalties. Even when Stanford seemed content to settle for a field goal, USC offered up an extra four points on a platter, lining up in the neutral zone at the end of one second-quarter drive and giving Stanford another chance on fourth down. The Cardinal complied, scoring a touchdown.

A pass that slipped through Drake London’s hands was intercepted and returned for a score. Two more penalties spurred another Stanford touchdown drive.