Narrated by an unnamed third-person narrator, "On the Bridge" opens with Seth Dawson, the story's protagonist, leaning on the stone wall of a bridge with Adam Lockwood, another schoolboy. Seth is struggling to smoke his first cigarette and observing the way Adam exhales through his nose.
Adam mentions that he beat up a taller, older guy the day before. Seth is impressed with Adam's casual manner and by the idea that Adam is known to police. Seth himself has never been in trouble with the law.
After a girl sitting in a convertible that passes under the bridge waves at Adam, Adam brags about going out with older girls all the time and making out with them. This sounds incredible to Seth, who has barely kissed a girl.
Adam successfully signals a truck driver to blow his horn, but when Seth tries the gesture, he is ignored. Adam makes Seth believe that the gesture has to be done in a special, secret way, which makes Seth feel inadequate and less cool than Adam.
When Adam pretends to throw a rock at a car, the driver almost loses control. Afraid that they will get caught by the police or anger a driver, Seth asks Adam to stop. However, Adam flicks his cigarette off the bridge and onto an oncoming car.
The car pulls off the exit ramp next to them and three men get out, surrounding the boys. The driver demands to know who flicked the butt. While Seth privately resolves to never tell on a friend, he realizes that Adam is pointing at him, suggesting that Seth threw the cigarette.
The driver grabs Seth and forces him to lick the cigarette ash off the windshield. Seth notices that Adam has disappeared. When Seth refuses to lick the ash, the man slams Seth's head against the windshield and gives him a bloody nose before driving away.
Adam reappears and tries to comfort Seth, who is bleeding. Adam claims he could not intervene because one of the men had a small knife on him. Seth realizes that Adam is lying and walks home alone. Before going inside, he stops at the curb and throws away his blood-stained denim jacket in a garbage can as a symbolic rejection of his efforts to emulate Adam's tough-guy posturing.