For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
We meet Eddie and May, two former lovers. Eddie has come to visit May in her motel room; it seems that they had been living together before, and May had peaced out when Eddie went on a long trip without her.
May seems ambivalent about Eddie being there. It seems that he's scooted off and left her over and over again, and she's pretty peeved with him for that. She seems to want him to leave… and, yet, she's also terrified that he will do just that. Of course, she's aware that she's expecting another gentleman caller that evening, so part of her wants Eddie to go to avoid awkwardness.
When Eddie realizes that May is seeing another dude, the tension between them really heats up. He does not like that news one bit, and at one point he goes to get a shotgun. It seems he wants to stick around to "meet" Martin (and potentially do him physical violence). Naturally, May isn't so into that idea.
Act II
While they're arguing back and forth about the boyfriend and their past relationship, a woman shows up and shoots out the windshield of Eddie's truck. May assumes this is a woman she knows as the Countess, who was Eddie's mistress at one point. So then May's livid about that.
Then, Martin (her date) shows up, and Eddie ends up launching into some details about their relationship. It seems that not only are they former lovers, they're also (half) brother and sister. Gross. Eddie and May each relay their memories of their father's bigamous shenanigans with their respective mothers and the impact his betrayal had on them.
Act III
Martin is apparently the sport of the century, so he sticks around to hear all this (okay, he tries to leave a couple of times, but he doesn't really try that hard). Meanwhile, Eddie and May get so riled up by the stories that they start making out.
And then (because this isn't already enough of a circus), the Countess shows up and sets Eddie's truck on fire. Eddie claims he is going out to check on the damage, but May knows that he's not coming back. So she packs a bag, ostensibly to follow him? Meanwhile, poor Martin is left standing there . . .