When a light changes from red to green and the truck in front of her doesn't move, Rachel (Caren Pistorius), a young mother with her teenage son (Gabriel Bateman) in the car, leans on her horn impatiently, then tears out from behind it.
When she's caught in a traffic jam up ahead, the man (Russell Crowe) in the truck pulls up beside her and asks her son if he knows what a courtesy tap is. He gives an example, tapping his car horn lightly and tells the boy, "I'm sure that's what your mom meant to do."
Still angry, she replies, "No, it's not." The man explains, "I've been having kind of a hard time lately, I'm sorry." He asks her to accept his apology and suggests that maybe she wants to apologize as well. Rachel tells her horrified son to ignore the man and says to the guy in the truck she doesn't have anything to apologize for.
Enraged, the man begins to follow Rachel. He tells her he doesn't think she knows what a bad day is and threatens, "But you're going to find out." He goes after Rachel, her family and everyone in the vicinity as he sets out to teach her what a bad day is and when to apologize, as he uses violence and retribution as his teaching aids.