The following version of this book was used to create the guide: Lacey, Catherine. Biography of X. Farrar, Straus, and Giroux, 2023.

In the wake of the death of her wife, an enigmatic artist named X, a woman named C.M. Lucca endeavors to write a corrective biography. She ostensibly undertakes this effort because she finds the extant biography about her, written by a man named Theodore Smith, insufficient and inaccurate in its rendering of her life's work. However, C.M. struggles to keep her personal feelings for X separate from the objective project of the biography, and the narrative thus takes shape as a mixture of personal reflection and diligent historiography. C.M. sets about tracking down X's familial origins, first traveling to a deer processing plant in Missoula, Montana before learning that X was born in Byhalia, Mississippi, a small town in what was formerly known as the Southern Territory. From the end of WWII until the mid-1990s, the Southern Territory (or the southern United States) existed as a separatist theocracy with an authoritarian regime.

X makes a trip to the Former Southern Territory and is shepherded by a tour guide named Nancy. She interviews X's parents, friends, and collaborators in the region and learns that they all believe X was killed during something called the Revelation Rifle Affair, an event in which X and several others helped bomb a munitions factory. C.M. is also unnerved to discover that X was married to a man named Paul Vine when she was a teenager, and begins to feel increasingly insecure about how well she knew (or did not know) her wife. Upon returning to the Northern Territory, C.M. becomes increasingly combative in her interviews as she discovers an entire private world that X never let her in on. She speaks to Ted Gold, one of the other migrants from the Southern Territory whom X worked with to orchestrate the Revelation Rifle Affair, and is disturbed to find that X used to call him her "biographer" in letters. She investigates a relationship X had with a folk musician named Connie Converse and becomes convinced that X's affection for Connie far surpassed her affection for C.M.. She also speaks with Oleg Hall, X's one-time benefactor and close friend, with whom she has always had a rivalrous relationship, and cruelly tells him that X manufactured their entire relationship for her financial benefit.

Further inquiries lead C.M. to the conclusion that X did not respond well when her relationship with Connie (which bordered on romance) fell apart. C.M. travels to Italy to locate an address in one of X's diaries and meets a woman named Gioia, whom C.M. slowly comes to realize was the romantic partner of a woman that X had an affair with in Milan immediately after Connie's departure. When C.M. returns stateside, further interviews with Bertha Hurts (an editor at X's lit mag), Ginny Green (X's most prominent curator) and Fred Holton (a man who claims X was his partner in the FBI) cause her to spiral further into a belief that her wife was not fully honest with her about her life. C.M. becomes increasingly bitter and lashes out at Ginny in much the same manner that she did at Oleg.

C.M.'s research leads her to a meeting with Marion Parker, X's first wife. It becomes apparent during their interview that Marion was abused both physically and emotionally while she was married to X, and their meeting prompts C.M. to begin reflecting on the dynamics of her own marriage. Pursuit of information about X's relationship with an artist named Alfred Schuster leads C.M. back to Oleg, who further weakens C.M.'s self-confidence by implying that X only got together with her because she was at a low point in her career. C.M. begins wondering whether she is a good journalist at all, and recalls her most important piece of reporting, an undercover project she was only able to undertake by imitating X's capacity for assumed identities and bold-faced dishonesty. Contemplation of this period of C.M.'s life leads her to reveal that, shortly thereafter, X bought her a cabin and trapped her in a subordinate, wifely role not so different from the one described by Marion Parker.

As the novel draws to a conclusion, C.M. increasingly recalls moments during her marriage when X was violent or abusive. She recollects an instance in which X raised a croquet mallet to a man's head, and then recalls that X once slashed open her own cheek as a means of making a statement. When C.M. travels to New Mexico after following instructions in X's private files, she is met by a woman named Shelley with whom X was clearly having an affair. Shelley takes C.M. to see X's final gallery project, which she never finished, and C.M. is horrified to discover a photographic exegesis of her own marriage which paints her in an unflattering and scientific light; the project is titled You Ruin Things. The trauma of the experience causes C.M. to recall a moment toward the end of X's life when she pointed a gun at C.M. for reasons C.M. could never quite discern. She ends the biography abruptly with an account of X's death and the way that both she and the media reacted to it: by believing that X was merely playing a prank on them.