British author James Miller (Shimell) finds himself in the picturesque landscapes of Tuscany, where he is scheduled to give a talk about his new book, titled Certified Copy. In this provocative work, he posits that questions of authenticity in art are inconsequential, asserting that every reproduction can stand as an original, and even the so-called original is merely a copy of another form.
During his engagement, he crosses paths with a French antiques dealer, who remains unnamed (Binoche), as she brings her 11-year-old son to the event in hopes of having several copies of Miller's book signed. However, her plans are disrupted when her son grows restless and hungry, prompting her to leave, though she thoughtfully leaves her phone number with Miller's interpreter.
The dynamics between the woman and her son reveal a strained relationship; he teasingly hints that she might have romantic feelings for Miller, which she promptly denies.
Their destinies intertwine further when Miller and the woman meet at her antique shop. Initially, she hopes to engage him in browsing the unique items she has collected, but he dismissively suggests that they embark on a drive, promising to return in time for his 9 p.m. train. In their meandering journey, Miller takes time to sign multiple copies of his book for her, including one for her sister, Marie, inscribing a note that brings her some distress. The two delve into a spirited debate about the crux of his book, where they explore the connections between authenticity and simplicity. Their discourse grows tense as Miller provocatively reminds her that even the Mona Lisa is effectively a "copy" of the woman who inspired it, be she real or imagined.
Determined to share more with him, the woman leads Miller to an art museum, showcasing another famed "copy," but he shows little interest, claiming his work is complete and he has lost enthusiasm for further examples. As the woman shares her frustrations about her rebellious son, Miller attempts to rationalize his behavior, unwittingly aggravating her deeper feelings of helplessness.
On his encouragement, they pause at a nearby café for a quick coffee. While Miller steps outside to take a call, the café owner mistakenly assumes he is her husband, prompting the woman to play along as he offers his insights on the significance of marriage and the need to accept each other's imperfections. When Miller returns, she shares this amusing misunderstanding with him, which alters the nature of their conversation significantly. They begin to blend French and English in their dialogue, affectionately referring to her son as "our" son.
She confides her disappointment over their lackluster 15th wedding anniversary, revealing that he had fallen asleep while she prepared. She confronts him about his frequent work trips, which have created emotional distance in their relationship. As they stroll through the enchanting town, they observe an older couple discussing a statue, leading an older man to provide unsolicited, yet well-meaning advice about rekindling affection within marriage.
Trying to connect, Miller places his hand on her shoulder, yet tensions rise again during dinner when Miller argues with the waiter over their wine. In a boiling confrontation, he storms out but waits for her outside. Their paths lead them past a church, where she chooses to enter alone, claiming it is merely to remove her bra. Intrigued, they follow two elderly churchgoers to their home next to the hotel where Miller and his wife shared their honeymoon, a memory that remains hazy for him.
Inside, the woman asks to see the room, igniting a moment of nostalgia filled with exchanged memories from their wedding night, though Miller struggles to recall the finer details. She implores him to embrace their flaws, cautioning that loneliness is the alternative to acceptance. Despite her heartfelt plea, he reiterates his need to catch his train. In a subdued moment of reflection, he enters the bathroom, staring at his reflection as the church bells toll 8 p.m., heralding the closing moments of their time together.
