All three of our main characters, the narrator, Digby, and Jeff, change considerably overnight in this story. In the opening of the story, they're all trying to be "tough," "bad characters." They think this means they have to dress a certain way, talk a certain way, hang out and get into mischief until dawn, drink and do drugs, and listen to rock and roll. The narrator keeps a tire iron under his seat because "bad characters always keep tire irons under the driver's seat." They have notions of what it means to be a "bad character," and they assume that being bad characters is cool and desirable.
They're play-acting, of course. They're all upper-middle-class or higher kids. Digby "allowed his father to pay his tuition at Cornell," a difficult school to get into, and Jeff is also in school, although he's thinking of quitting. They're all living the movies in their minds, including Digby, who has had a semester of "martial arts for phys-ed credit" and thinks he's Bruce Lee.
They cause the situation themselves by thinking it would be cool to interrupt Tony Lovett--whose car they thought they'd found at Greasy Lake--having sex with his girlfriend. They realized too late that it wasn't Tony's car and that they'd upset a man who was a truly bad character. Digby's martial arts attempt got him punched in the face, and the bad character would have beat all three of them had not the narrator had the tire iron; during the fight, however, the narrator at least realizes that he's terrified--quite the opposite of being a bad character. After dropping him with it, they assume they've killed him, which is also a rude awakening.
Next, the bad character's girlfriend emerges from the car and they turn on her. They think they've just killed a man and "she's already tainted," so they try to pin her to the hood of the car to rape her. This is when another car swings into the lot, and they run. The narrator runs into the weeds and the muck of the pond and is about to start swimming for the other shore when he bumps into what is undoubtedly a corpse floating in the water. This seems to be his turning point. The girl and the other men and the greasy character (who wasn't dead after all) beat his mother's car (also not very cool, driving your mother's car) and leave.
He and his friends finally come out of the woods when the sun comes up, clean some of the junk out of the car, find the keys, and are about to drive away when they get an invitation they wouldn't have refused a few hours before. Two older girls come up looking for Al (probably the dead man in the lake) and when they can't find him, offer the boys in the car, "Hey, you want to party, you want to do some of these (pills) with me and Sarah?" She even calls them "bad characters," which before the events of the evening would have been the ultimate compliment.
The narrator says, "I just looked at her. I thought I was going to cry."
In the course of one evening, they've been beaten and shocked awake. They aren't "bad" and they don't want to be bad. They've had a glimpse of what it really means to be "bad," and they're finished. They've just learned that life isn't the movies. In encountering the corpse, the narrator undoubtedly realizes that that could have easily been him, and in the harsh light of day, the girls taking drugs aren't attractive and it suddenly doesn't look like fun.