Marc Thiriez, a middle-aged man from Paris, finds himself at a crossroads when he casually inquires of his wife, Agnès, if he should shave off the moustache that has been part of his identity for most of his adult life. Agnès, portrayed by Emmanuelle Devos, wittingly remarks that she would hardly recognize him without it. Nevertheless, as soon as she steps out, Marc decides to take the plunge and shaves it off. When Agnès returns home, she expresses her frustration that he didn’t let her in when she rang the doorbell. In a desperate attempt to garner her attention with his newly clean-shaven visage, Marc is met with indifference.
While driving to a party with Agnès, Marc eagerly asks if she notices anything different about him, but she deftly redirects the conversation. Upon arriving at their friends' home, the couple encounters Serge, Agnès' ex-lover, and his partner Nadia. During conversations filled with banter, Serge shares anecdotes about Agnès, highlighting a time when she was deceitful yet refused to acknowledge her misdeeds. Agnès firmly denies ever lying, elevating the tension. Feeling invisible, Marc grows agitated when Serge and Nadia fail to notice his clean-shaven face, leading to a heated argument with Agnès as they head home. Agnès, clearly shaken, expresses her concern for Marc's well-being during their nightly confrontation in bed, insisting he has never had a moustache.
In his mounting confusion, Marc discovers a treasure trove of their vacation photographs from Bali, each snapshot showcasing his signature moustache. Confronted with this evidence, Agnès sidesteps the issue entirely, further amplifying Marc's paranoia. As the days pass, not even his colleagues acknowledge Marc's transformation, driving him to dig through the trash in search of his discarded moustache hairs, which only terrifies Agnès more. Concerned for his mental state, she suggests seeking help from a psychiatrist known to her friend, François. Agnès goes on to buy Marc a garish jacket that he unflatteringly refers to as "a clown's jacket."
During a photo session for a work badge, Marc urgently asks a female photographer if she can notice any difference between his new and old photographs. Though she comments on his moustache, he remains unaware of his deteriorating mental condition as he interacts with Agnès' friend Bruno, who informs him that he hasn’t had a moustache in over a decade. When Marc converses about making plans to visit his parents for dinner, Agnès gently reminds him that his father has been deceased for a year, deepening his sense of bewilderment as he references Serge and Nadia, who Agnès claims she does not know. Lost and distressed regarding his grip on reality, Marc struggles to cope, and with Agnès' administration of a sleeping pill, he goes to bed.
In a shocking twist, Marc awakens to overhear Agnès and Bruno discussing the possibility of committing him to a psychiatric facility. Acting swiftly, he dresses and escapes. Attempting to retrace his roots, he jumps into a cab hoping to locate his childhood home, but the relentless rain and the effects of the sleeping pill complicate his search. After a series of futile attempts to reconnect with his past, he makes a dubious call to Agnès, falsely claiming to be at his mother’s house, imploring her to come and collect him. Upon witnessing Agnès and Bruno exit his apartment, he hastily grabs his passport, only to find himself fleeing to Hong Kong on the first available flight—even as his passport photo still depicts him with the moustache he had shunned.
In Hong Kong, Marc pens a postcard, intending to apologize to Agnès but ultimately refrains from sending it. Just as he gears up to return to France, he has a change of heart and spends an entire day on the Star Ferry. When the ferry halts operations for the night, he finds himself chartering a boat with local fishermen to an unnamed village in China, eventually embracing the locals and allowing his moustache to flourish anew.
Having spent time away, Marc returns to his hotel one day to an unexpected sight—Agnès waiting for him as if she had been with him all along. Surprised and bewildered, he witnesses her playful jabs about his flashy jacket, interrogating him about why he purchased it, and she fervently hopes he won't wear it in Paris. Their casino visit introduces him to new friends, all of whom Agnès seems to know intimately but with whom Marc feels disconnected. To his annoyance, photos surface that show Marc still outfitted with his moustache. Sensing his discomfort, Agnès casually suggests they can simply avoid these new acquaintances when back in Paris if he prefers.
In a quiet yet profound moment back at the hotel, Agnès proposes that Marc ought to shave his moustache once more so he can show her his face without it at least one time. Following through with her request, Marc complies, and while she admires his appearance sans the facial hair, her words echo a deeper sentiment on identity and perception that resonates profoundly both within him and amongst their shared experiences.
