There's nothing particularly dramatic about how Amy found out about Nick's affair. While home alone one night, she decided to surprise him at The Bar and spend a romantic evening with him—nothing kills your desire for a romantic evening though, quite like seeing your husband leave a bar with another woman and then kiss her under a tree. Amy followed them to Andie's apartment and then sat on the front steps trying to plan her next move; eventually she got too cold (because it was snowing) and went home.
Amy isn't one of those girls who confront their cheating significant others in fits of rage. No—what she does is worse than that. She keeps the knowledge to herself and lets it fester and infect her. Her anger over Nick's lies after everything she's sacrificed for him makes her feel worthless and stupid; in her eyes, the only solution is to rewrite history to make Nick the bad guy and restore her to being Amazing Amy, rather than "Average Dumb Woman."
Maybe you think Amy's being a little melodramatic and extreme by deciding to frame Nick for murder. Maybe she should have exploded in a fit of rage and left him. To you skeptics, Amy says no way—she refuses to be treated like a doormat and will have her revenge. Nick must be taught once and for all that he's not entitled to everything, that he isn't the golden boy—and if it takes prison and a lethal injection to do that, then so be it.
That said, Amy continues to describe what really happened the morning she disappeared, as well as let us in on more of her brilliantly twisted plan. To begin with, she bought a very ordinary, very forgettable used car on Craigslist, drove to Arkansas to pay for it and take it home, and has been parking it in long-term lots in St. Louis for five months. From this point on, her plan is to stay at an isolated campground with cabins in the Missouri Ozarks (with cable television, of course) and watch Nick self-destruct.
She also stops at a truck stop and cuts and dyes her hair in the bathroom. By her description, she's gone from being a super stunning blonde to an ordinary looking girl with mousy brown hair and glasses; it occurs to her that Nick never would have been attracted to her in the first place if she'd looked like this when they met.
But what about Amy's diary? She seemed like such a funny, sweet, cool girl in those entries, so who the heck is this vindictive and super bitter lady? Here's your answer: Diary Amy doesn't exist. Amy had to create a likeable, devoted, and even overly trusting persona in order to win sympathy from the cops and the viewing public. All the entries point to one conclusion: that Amy loved Nick, but was also terrified of him. Hence all the entries ("If you read this and I'm dead") that point to Nick as a murder suspect.
You almost have to admire Amy's resolve and attention to detail with this whole thing—she even went back through all her planners and researched current events to preserve the authenticity of each of the seven years of entries.
Amy knows her parents are pretty freaked out, but frankly, she doesn't really care. To them, she's always been Amazing Amy, their business, not their daughter, so she figures they deserve to think she's dead after objectifying her all those years. The same goes for Nick. To Amy, the best thing about this plan is that everyone gets punished.