Darcy Anderson has been married to Bob, a Portland, Maine, accountant, for 27 years. They also have a mail order business selling and appraising rare coins. But one night, while Bob is away on business, Darcy goes into the garage to search for batteries. When she rummages through Bob's belongings, she stumbles across a pornographic magazine showing images of sadomasochism. Unnerved by the magazine – and the fact that it is in Bob's possession – Darcy finds a secret compartment behind the garage's baseboard and makes a more horrific discovery: a small box containing the ID cards of Marjorie Duvall, a victim of a serial killer called "Beadie". Bob calls Darcy and senses her distress; she lies about the reason for her anxiety.
Afterwards, she googles "Beadie" and cross-checks Bob's business records with the locations of the murders, finding that Bob was within the proximity of most of the crimes. Overwhelmed, Darcy falls asleep. However, when Darcy wakes up the next morning, she finds that Bob has deduced Darcy's discovery and returned home early. He proceeds to calmly explain his insanity to a horrified Darcy, recounting how he and a sadistic friend named Brian Delahanty – nicknamed "BD", from which Beadie's name was derived – planned a school shooting as teenagers. Delanhanty was hit by a truck before they could carry it out, but Bob claims he had "infected" him with "certain ideas", resulting in his homicidal urges. Bob claims that after Darcy married him and helped raise his children, his murderous alter ego never drove him to kill again for several years. He pleads to Darcy to put the matter behind them, for the sake of herself and their family. After mulling it over, Darcy feigns an agreement to do so, on the condition that he bury Duvall's ID cards behind their house. Bob believes Darcy has put the truth behind her, when in fact she is trying to think of a way to stop him from killing again.
A few months after Darcy's discoveries, an elated Bob finds a rare 1955 doubled-die cent, and the couple goes out to Portland to celebrate. When Bob becomes drunk from champagne, Darcy devises a plan to murder him. Upon arriving home, Darcy has Bob fetch some Perrier while she waits for him upstairs, ostensibly for sex. However, when Bob arrives, Darcy pushes him down the stairs, breaking his arm, neck, and back. She then manages to shove a plastic bag and a dishwiper down his throat, killing him. Darcy manages to convince the authorities and the children that Bob had died in a drunken accident, and isn't suspected of committing any foul play. Darcy assumes the ordeal is over.
However, not long after Bob is buried, a retired detective named Holt Ramsey visits the house. Ramsey investigated the Beadie murders and had questioned Bob after the death of another victim, Stacey Moore, who worked at a restaurant that Bob frequented on his business trips. Ramsey tells Darcy that he suspected Bob for the murders, since his Chevrolet Suburban was seen in the neighborhood of each victim. Darcy realizes that Ramsey has figured out her role in Bob's own death. Once she admits the truth, Ramsey assures her that she "did the right thing" and leaves; before he does, she tells him about Delahanty. Darcy realizes how Bob was close to being caught and wasn't as smart as he thought he was. She also finds that she can now be at peace with herself.
A 2,581-word preview was made available by Simon & Schuster prior to the story's publication.