As hannahschychuk points out, the entirety of Atonement is Briony's atonement--though the novel is by Ian McEwan, the internal world of the novel belongs to Briony. The story is told through her perspective to exonerate Robbie, who she accused of raping Lola. By the end of the story, dated 1999, we realize that what we've been reading is a text 64 years in the making, not just an abstract piece of fiction.
What makes this metafiction, and therefore postmodern, is that the novel acknowledges that it is a written text. By acknowledging that it's a written work, Atonement is able to ask different questions and inspire different responses than if it were simply the straightforward story of Briony or Robbie or Cecilia. Instead, it is that story, but it's also showing Briony's development, her change of heart, and her attempt to atone for ruining a young man's life. Because we read this from her perspective, we have a different view on the events than if we read solely from Robbie's point of view, or even a third-person narrator. Though McEwan is the author, acknowledging that it is a written work within the text gives us an extra element to consider in our interpretations, namely, the relationship between fact and fiction and how Briony's truth has evolved and shifted.
This comes up frequently in the novel, in quotes like the following:
“It wasn't only wickedness and scheming that made people unhappy, it was confusion and misunderstanding; above all, it was the failure to grasp the simple truth that other people are as real as you.”
“Was everyone else really as alive as she was?...If the answer was yes, then the world, the social world, was unbearably complicated, with two billion voices, and everyone's thoughts striving in equal importance and everyone's claim on life as intense, and everyone thinking they were unique, when no one was.”
“How can a novelist achieve atonement when, with her absolute power of deciding outcomes, she is also God? There is no one, no entity or higher form that she can appeal to, or be reconciled with, or that can forgive her. There is nothing outside her. In her imagination she has set the limits and the terms. No atonement for God, or novelists, even if they are atheists. It was always an impossible task, and that was precisely the point. The attempt was all.”
All of these quotes explore the relationship between a writer and the world, and, more specifically, between the writer and her work. Over the course of writing her atonement in the form of this novel, Briony also comes to see that this is a story she has manufactured from the beginning--the narrative of Robbie assaulting Lola--and that her viewing others as nothing more than empty characters waiting for her to complete them has been hurtful. In the final quote, she comes to the conclusion that nobody can forgive her--what she's done, what she's written, is finished. All she can do is manage the consequences.
Postmodernism as a genre and method of thinking critiques the common concepts of art; in the case of Atonement, it questions reality, narrative, writing, truth, and innocence. All of these things are inherent because of the metafiction, which centers it firmly as a postmodern narrative and allows it to interrogate these concepts through the fictional novel, the "true" story within, and the various ways that Briony has shaped and twisted the truth to suit her version of the events.