Verity opens in contemporary New York City, as protagonist Lowen Ashleigh witnesses a bloody accident where a passerby is hit by a truck. Lowen, a struggling writer, is grieving her mother's recent death and wondering where her foundering career is headed. She writes thrillers, but she’s suffering from intense writer’s block, which her state of uncertainty only seems to worsen. She’s so numbed to reality that she barely registers the accident, even as she’s soaked with the victim’s blood. 

To her surprise, a stranger helps clean her up, and they share a moment of intense romantic chemistry. She soon discovers his name is Jeremy Crawford, and that he’s married to bestselling author Verity Crawford. This author—whom Lowen respects enormously for her meticulous research and well–crafted plots—has been left incapacitated by a car accident. Verity is an internationally renowned writer, who is best known for her tactic of writing novels from the perspective of the villain. In a meeting at Pantem Publishing, Jeremy offers Lowen a significant sum to finish Verity’s final series of novels. Lowen’s agent Corey encourages her to take the deal, but Lowen is extremely nervous about the prospect. However, she can’t refuse the enormous sum the Crawfords propose to pay her. She accepts the offer with some trepidation. The narrative then moves to Vermont, the location of the Crawford family home. At Jeremy’s invitation, Lowen moves in temporarily to sort through Verity’s office and get a feel for her writing.  

However, while trying to research Verity’s work, she discovers an unpublished autobiography of Verity’s that unveils startling secrets about her life. This unpublished manuscript, entitled So Be It, disrupts the kind and loving façade that Verity has been painstakingly keeping up for over a decade. After this point, Lowen’s perspective in the novel is occasionally replaced by Verity’s as chapters of this autobiography appear in full. Rather than the loving mother and wife everyone else apparently knew, it paints Verity as a selfish and manipulative person with little regard for anyone else’s feelings. Because Verity lies comatose upstairs as Lowen reads this, Lowen begins to feel extremely uneasy about her discovery. She decides to keep the autobiography hidden from Jeremy, especially when she discovers that it reveals information that connects Verity to the tragic deaths of the Crawfords' twin daughters, Harper and Chastin. 

Lowen soon settles into life in Vermont and begins to build a relationship with Jeremy and Crew, his only living child. It’s a double life, as she grows emotionally attached to Jeremy while attempting to avoid the judgment of the Crawfords’ neighbors and nurses. She’s also concurrently and secretly following the chilling revelations that unfold constantly as she gets further into So Be It. The more Lowen becomes consumed with Verity’s dual existence, the more intensely she feels a romantic connection to Jeremy. At this point in the novel, Jeremy has only acted respectfully, unselfishly, and chivalrously in every interaction Lowen has seen. Although she knows he’s married, she can’t shake her attraction to him. The two draw closer and closer as their attraction to each other becomes too strong to ignore. Financial circumstances and an apartment offer falling through mean Lowen has to stay in Vermont much longer than planned. This gives her time for two things outside of her work: to get to know Jeremy and to develop a building horror of Verity’s presence and the unnerving coincidences that seem to follow her. Lowen quickly begins to feel that she’s being carefully watched. 

Lowen begins to write Verity’s remaining books in earnest, sending outlines to her agent and trying to make sense of her mass of papers and notes. However, the temptation to covertly read more of Verity’s autobiography is strong whenever Lowen is alone in the office. Every time she succumbs to it, she discovers more unnerving things about Verity’s sociopathic, transactional view of relationships. She discovers that Verity is an obsessive, jealous lover who tried to abort her own twins when she realizes she would have to share Jeremy’s love with them. It also quickly becomes evident that Verity was directly involved in the murder of her daughter, Harper. She believed Harper had deliberately killed Chastin, even though there was no proof, o Verity killed Harper. Lowen is caught in a moral dilemma because this evidence had previously remained totally concealed. She struggles with whether it would be fitting or even ethically correct to expose Verity’s secrets to Jeremy, or if it would be kinder to protect him from the painful truth about his homicidal, mentally unstable wife. 

While all this is happening, the atmosphere in the Crawford home is becoming increasingly tense and mysterious. Lowen feels she is constantly being observed and begins noticing inconsistencies whenever Verity is left alone. Doors are inexplicably locked and unlocked, household objects disappear, and devices mysteriously turn off or are set to mute. Lowen is sensitive to small coincidences and to the privacy that locked doors promise because she has a history of dangerous sleepwalking. She suspects that Verity can move around and isn’t actually comatose at all. This is later reinforced by Crew, who accidentally mentions things his mother has recently said to him. Despite all this, Lowen's feelings for Jeremy deepen, and they begin a romantic, sexual relationship. This all contributes to the novel’s climax, where Lowen reveals the autobiography to Jeremy. Jeremy doesn’t want to believe Lowen when she tells him Verity is not immobilized, but he eventually relents and agrees to read So Be It. After Lowen gives him the autobiography, she waits in agony for him to finish it. She hears him cry out in pain and storm upstairs to confront his wife. 

Verity pretends to have no reaction when Jeremy speaks to her and only responds when he threatens to hand her manuscript over to the police. She drops the mask of being unconscious and reveals she’s been aware of everything that’s happening in the Crawford house. She’s been pretending to be comatose while manipulating the situation to test Jeremy's loyalty to her, and to try and escape with their son Crew and the funds from Lowen’s new books. Jeremy, furious, tries to strangle her for killing Harper. Lowen stops him, telling him that if he kills her that way, Crew will lose his father. She suggests instead that if Jeremy forces her to vomit, they can make the murder look like inhalation pneumonia and avoid a criminal investigation. Jeremy does so, and Verity dies gruesomely, choking on her own vomit. 

At the end of the novel, Jeremy, Crew, and a pregnant Lowen are in the process of moving out of the Vermont house to start a new life in North Carolina. However, Lowen finds a letter addressed to Jeremy hidden under the floorboards in her room. In this letter, Verity makes shocking revelations. She reveals that she’d written the autobiography first as writing practice and then as a therapeutic exercise, casting herself as the “antagonistic narrator” as she did in her novels. The letter says that she had planned to escape with Crew and explains that Harper’s death was actually an accident. She says she still loves Jeremy and regrets what happened but must prioritize Crew’s safety. Lowen panics and destroys the manuscript, deciding never to burden Jeremy with the knowledge of it.  

However, she’s still unsure whether the letter or the autobiography reflects the absolute truth. The novel ends with Lowen questioning the extent to which any of the people involved in the Crawfords’ family tragedies understood the whole of the matter and acknowledging the ingenious manipulations Verity was able to orchestrate.