Atonement's epilogue delivers a big revelation: after the third-person narrator gives way to Briony's first-person perspective, the reader discovers that she has actually written everything until this point. With this epilogue, McEwan transforms the narrative into a kind of frame story, making the reader retrospectively embed the novel's three constitutive parts within the context of Briony's composition of them.
The title of the epilogue is "London, 1999." Fast-forwarding from the middle of WWII until the end of the century (over 50 years after the latest events of the story), the epilogue opens on the morning of a first-person narrator's "seventy-seventh birthday." This involves a notable shift in the narration. Until this point, the novel has been narrated by a third-person narrator, whose focus in each chapter shifts between various characters. Although certain details incline the reader to believe that this first-person narrator is Briony, it takes several pages before McEwan makes it absolutely clear. Until this point in the novel, Briony is already one of the characters that the narrator has focused on most. When Briony moreover becomes the only character with the opportunity to speak directly to the reader, McEwan advances her authority further.
Briony's authority surges as the reader gradually, over the course of the epilogue, comes to learn that she is the novel's writer. McEwan hints at this over the course of the epilogue, but spells it out in the final pages. After her birthday part, Briony sits at the writing desk of her hotel room, thinking of her "last novel," which she thinks should have been her first. Half a dozen drafts separate the earliest version of this novel (from January 1940) from the latest (from March 1999). She reflects that her "fifty-nine-year assignment is over," and that she has "regarded it as [her] duty to disguise nothing—the names, the places, the exact circumstances." Through these reflections, McEwan asserts that the novel features a fictional writer from within its fictional world.