Wanting his high school crush Martha (Wayne) all to himself, Professor Rusty Darling snatches a drop of her blood and decides to clone her with technology he has recently developed. However, his assistant, Hank Dean (he and his wife being unable to have a child of their own and wanting a boy) manipulates the gene coding process so that the clone will be born male. When Prof. Darling witnesses the birth of the boy, he fires Dean and leaves the baby on his sister Charity’s doorstep. Darling is arrested a week later due to evidence of the cloning provided by Dean. Charity names the boy Jack Darling and legally adopts him as her own. Jack grows to be a very intelligent young boy and, by the age of ten, already excels in math, physics, and particularly chemistry. For Jack’s tenth birthday, Charity buys him a deluxe chemistry set. Using the Darling family wealth, Jack has his mother order him proper chemicals with which to experiment; Charity does this reluctantly. Mixing extracts from rare red and black flower petals in a beaker of water, Jack, still being a naïve child, unintentionally manufactures a poison that brings about uncontrollable laughter and eventual death. Jack escapes the fumes, but his mother becomes trapped in the house and the toxin fills her lungs. When the paramedics arrive, they bring Charity out on a stretcher; Jack demands to look under the cloth, and he bursts into tears when he sees the toothy grin he’s left on his mother’s face. Jack is then sent to an orphanage, where he spends a little over six years before he is adopted by the Lennons. Jack, keeping the name Darling, finds himself humbled by his new family’s poor finances. Marcus Lennon is a comedian-turned-schoolteacher, Mary Lennon is a stern pastry chef, Chuck Lennon is the star of the high school football, basketball, and baseball teams, and Holly Lennon is a rebellious teenager that is Jack’s age, 16. This new family, particularly Marcus and Holly, heal Jack’s emotional wounds. Marcus teaches Jack the art of comedy with his constant joking, and Holly, for worse, turns Jack into a foul-mouthed rebel like herself. Since there is no blood relation, Holly and Jack become romantically involved, often sneaking into each others rooms at night to fornicate. When Mary walks in on one of these sex sessions, she grounds Holly and Jack, putting locks on their doors. Holly decides to run away, but Jack, still valuing his education, decides to stay with the Lennons. Jumping forward, Jack, having given up his rebellious ways, graduates as the valedictorian of his high school class and pursues a career in chemistry and education at a nearby university. During his last year at the university, Jack receives news that his parents and his brother, who were on their way to visit him, have been killed by a drunk driver. Having enough credits already to graduate, Jack leaves the university and uses what’s left of his inheritance from Charity to pay for his family’s funeral and to pay off their debt. Once this is done, Jack takes a job as lab assistant at Ace Chemical Processing where he meets Jeanie (the supervisor’s secretary), who he later marries. After five years at Ace, Jack decides that he just doesn’t have any interest in it anymore and quits, becoming a comedian. People just don’t find Jack that funny and he becomes an emotional wreck. Jack’s income drops and Jeanie is out of work with a baby on the way. The young couple moves into a duplex, rented out by Mrs. Burkiss. This is when the events of “The Killing Joke” play out and Jack, having gone through a life of tragedy, loses his mind, forgetting who he is, and becomes Joker.
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Throughout the book, the story will flash forward to the present, where a serial killer is plaguing Gotham City; this killer turns out to be Holly Lennon, who Joker recognizes, but doesn’t remember. This present side story has Batman and Joker both hunting down the killer; Batman wants to bring the woman to justice and Joker wants to figure out who she is. Joker finds the woman first. Holly somehow recognizes Joker as Jack, despite his deformities. Upon hearing her mention his true name and seeing Holly’s face brings all of his old memories rushing forward. This drives him mad and he viciously stabs Holly to death with a knife that he steals off of her while he kisses her. Batman stumbles upon this scene and Joker simply remarks that he’s done Batman’s work for him. Joker drives down to suburban Gotham where the Lennon home now sits condemned. Batman follows Joker and asks, “What is this place?” Joker replies, “I don’t remember.”