Paul, a young idealist recently discharged from military service, embarks on a search for employment. While at a café, he encounters Madeleine Zimmer, a young singer aspiring to record her music. Their day takes a dark turn when they witness a heated argument between a woman and her partner, which escalates to the point where the woman pulls out a gun and fatally shoots him. Following this shocking incident, Paul meets up with his friend Robert Packard, a journalist who persuades him to sign a petition advocating for the release of several artists and writers in Rio de Janeiro who have been accused of opposing state policies.
As Paul secures a job at a magazine, he has an awkward confrontation with Madeleine in a bathroom, where he expresses his hope for an outing that evening. However, she challenges his notion of "going out," leaving him speechless, and reveals that her earlier promise was insincere. With the French presidential election of December 1965 approaching, Paul turns 21 and finds himself increasingly drawn to Madeleine. She introduces him to her roommate, Elisabeth Choquet, and they also connect with Catherine-Isabelle, whom Robert is infatuated with. Paul joins Robert in pasting posters around Paris and expresses his protest against the Vietnam War by painting “Peace in Vietnam” alongside a U.S. Army vehicle.
Madeleine is gearing up for the release of her debut single under RCA Records. On the big day, Paul takes her to their café, intending to propose, but Madeleine, preoccupied with the release, postpones the conversation about their future. He records a heartfelt message for her in a coin-operated booth, only to be approached by a man who unexpectedly stabs himself. During a visit to a laundromat, Paul shares his unsettling sense of being followed with Robert, who, engrossed in a newspaper, passionately discusses folk singer Bob Dylan, whom he refers to as a "Vietnik."
Life takes a turn as Paul moves in with Madeleine, Elisabeth, and Catherine, ultimately leaving his magazine job to work as a pollster for IFOP. He interviews a young woman named Elsa, broaching subjects that include politics, romantic relationships, and contraception. In a candid moment at the café, Paul informs Elisabeth of Madeleine's pregnancy, although she expresses skepticism about the news. They later have an unexpected encounter with the woman who shot her partner, now involved in sex work, and witness a theatre director directing an actress. The group then attends a cinema where Madeleine professes her love to Paul. Excited yet compelled to act, Paul steps out to spray-paint an anti-Charles de Gaulle slogan on a nearby wall, prompting insights from Robert regarding the revolutionary role of workers.
A shocking event occurs when a man borrows matches from Paul, only to use them in a self-immolation act, leaving behind a haunting message that reads "Peace in Vietnam." Paul and Catherine visit Madeleine in the recording studio during her latest session, but she appears emotionally detached. After an encounter with a reporter, Madeleine requests Paul to get a car on her behalf. Embodying a military general's persona, Paul demands a car from the war ministry, and they depart together.
As the months pass from January to March 1966, Paul persists in conducting opinion polls focused on politics and relationships. However, he comes to realize that his subjectivity impacted the sincerity of the responses he received. At a police station, Catherine recounts an incident where Paul is said to have purchased an upscale apartment. Tensions rise as Madeleine expresses a desire to have Elisabeth move in with them, a proposal that Paul firmly opposes. Ultimately, reports indicate that Paul may have fallen from a window, raising questions among those close to him about whether it was an accident or suicide. Meanwhile, a pregnant Madeleine faces uncertainty about her future path as she speaks with authorities.
