Jane Fonda portrays herself as a Pharmacy customer and narrator in this intriguing tale. The life of Douglas "Doug" Varney, played by Sam Rockwell, a methodical pharmacist, takes a wild turn when he embarks on an affair with the alluring trophy wife of a customer. This whirlwind romance leads him down a treacherous path filled with sex, drugs, and the looming specter of murder.
Doug's personal life is far from perfect; he is married to Kara Varney (Michelle Monaghan), and their relationship is strained. Doug has dedicated years to running Walter Bishop's pharmacy, owned by Kara's father, played by Ken Howard. However, after Doug buys the business, he is frustrated that it retains the original name, which he feels undermines his efforts. Further complicating his life, Doug's family, including his own son, shows little respect for him. Meanwhile, Kara, a competitive cyclist, frequently belittles Doug in front of her friends, showcasing her superiority in the sport.
Despite being a conscientious pharmacist who meticulously provides the correct medications to his small-town clientele and manages to tolerate his unreliable employee, Noah (Ben Schwartz), Doug's complacent life is about to change dramatically. During a delivery to the home of Elizabeth Roberts (Olivia Wilde), the beautiful and troubled wife of Jack Roberts (Ray Liotta), Doug finds himself drawn into her world of drug addiction. When she can't pay for the delivery, she attempts to seduce Doug, leading to a passionate encounter.
As their relationship develops, Doug confesses that he once aspired to become a doctor but forfeited that dream when Kara became pregnant. Their affair intensifies, with Elizabeth teaching Doug about style and intimacy, while he experiments with pharmaceuticals in search of happiness and fulfillment. Doug's escapades become reckless as he mixes drugs for heightened experiences, embracing the notion of “better living through chemistry.”
Driven by a mutual desire to escape their mundane lives, Doug jokingly suggests that Elizabeth could poison Jack by tampering with his medication. However, she takes the suggestion seriously, complicating their already tangled relationship. When DEA Agent Andrew Carp (Norbert Leo) arrives to review Doug's inventory, Doug manages to keep his cool, staving off any immediate repercussions.
Trouble escalates when Doug's son Ethan is suspended from school, prompting Doug to take charge and bond with him. In a moment of rebellion, they deface the pharmacy’s sign, reclaiming Doug's identity. Determined to prove himself, Doug creates a drug that enhances his performance and challenges Kara during a cycling competition, claiming victory and rekindling their intimacy.
However, Agent Carp's second visit brings renewed anxiety as discrepancies in Doug's inventory are scrutinized. In a desperate bid to escape, Doug plots to overdose Jack with a heart medication switch, but in a twist of fate, he discovers remorse over the plan when he connects with Jack, only to find out he’s unintentionally put him in danger.
Chaos unfurls as Doug learns Noah has overdosed, inadvertently clearing him of guilt. The shocking news punctuates Doug's life, revealing that Jack is still alive. In the end, as Elizabeth prepares to leave town to avoid suspicion, Doug decides to stay for his son, leading to a mutual understanding and a divorce from Kara. Thus, Doug embarks on a journey of self-discovery, entangled in a web of chaos yet emerging with newfound clarity.
