Jeanne (Maria Schneider) is a 20-year-old Parisian woman wondering the streets looking for an apartment to live for herself and her fiancée. She goes alone to look at an apartment in a region of the Left Bank. There, she runs into a middle-aged, obviously depressed man (Marlon Brando) who is also looking at it. Jeanne quickly identifies him as an American because of his thick accent while they are communicating in French. After a few minutes of silence and some quiet dialogue, he picks her up, backs her against a wall, rips off her panties and has sex with her in the deserted apartment. They are very passionate yet never introduce themselves. When it is over, they each go their separate ways.Jeanne goes to the Paris Metro train station to meet her fiancée Tom (Jean-Pierre Leaud), a young television documentarian who has just returned to Paris following an assignment. They walk back to Jeanne's apartment where she currently lives with her mother having taken care of her since her father, a soldier in the French Foreign Legion, was killed in action in 1958 in Algeria. Despite their genuine loving engagement to be married, Jeanne is unhappy and annoyed that Tom insists on doing a cinéma vérité documentary all about her life and of her father, who is a well-known war hero. He films everything she does.Meanwhile, the middle-aged American man, who is named Paul, returns to a flophouse hotel in a run-down part of the city where he ventures to his apartment and watches a maid wash blood off the bathtub and surrounding walls. The maid speaks to Paul about the fact his French wife, Rose, two days earlier committed suicide in the bathtub by slashing her wrists. Paul doesn't say much and goes to his room to be by himself.The next day, Paul and Jeanne rendezvous back at the vacant apartment. Paul lays out ground rules that they are never to reveal their names to each other and never to discuss the outside world or to talk about their pasts. They are to meet there twice a week only for sex and tell no one about each other. But Jeanne (who has some knowledge of English after overhearing Paul talking to himself about his miserable life) keeps trying to find out who the strange American man is. Paul is adamant and refuses to talk about anything for he only wants to have sex with her and nothing else. Jeanne, unhappy with her own personal life, agrees to their arrangement. They lease several items of furniture (couch, chairs, table, bed, etc.) and bring it to the apartment which Paul signs a lease under a false name.The following day, Paul meets with Rose's mother (Maria Michi) to talk about the funeral arrangements. Paul soon becomes furious with her, screaming and cursing at her in a mixture of English and French, after she tells him she wants Rose to have absolution from a priest. Paul does not think that a person who committed suicide should receive absolution because (in his mind) killing oneself is a mortal sin.Meanwhile, Jeanne takes Tom and his camera crew on a tour of her former childhood house and shows him all her antiques. She will not tell him about her very first love who was her cousin. Ironically, Jeanne's cousin was also named Paul (not the same man.)Back in the apartment, Jeanne and Paul meet again where Jeanne wants to talk about her cousin, but Paul angrily stops her, reminding Jeanne of their agreement not to reveal anything, not even about what she did during the day. However, Paul soon relents and agrees to let her briefly talk about her cousin on the condition that no names are to be used. Jeanne talks about her cousin being the first man that she ever had sex with and that it was awkward, but nice. Paul then talks about his youth in the American Midwest and his unhappy childhood. Jeanne is proud she got some information about him, but Paul hints that he wasn't telling the truth. Jeanne becomes angry and asserts that Paul is an egoist and correctly assumes that something terrible happened in his life to explain his need to vent his anger and rage by having a meaningless sexual relationship with her. Paul will neither deny nor admit that Jeanne is right about him. Fed up with his arrogance and egotism, Jeanne masturbates.Another day or so later, Paul visits Marcel (Massimo Girotti), a tenant who lives in the hotel/flophouse that he and Rose managed and operated. It turns out that Marcel was Rose's secret lover. Paul doesn't know what she saw in Marcel or why she killed herself.Jeanne continues to get angry with Tom over filming her wherever she goes. After another argument with Tom, she returns to the vacant apartment to meet with Paul again, wanting to have sex wit him. She is shocked when Paul sodomizes her with a stick of butter, and forces her to renounce the Holy family, the Catholic Church, and all that broke her will as a child. It is painful for her, but she obeys.Another day later, Jeanne is in her wedding dress with her mother and various relatives and guests at the wedding rehearsal. Jeanne becomes more frustrated and insecure with everything happening around her and she leaves the rehearsal and returns to the vacant apartment, still wearing her wedding dress to meet with Paul again. She tells him that she couldn't leave him; although she is engaged to be married to someone she insists she loves while she does not love Paul. After helping her change out of her dress and bathing her, Paul tells Jeanne that true love in this world doesn't exist. To find her true love, Paul says, she "must go up into the ass of death". He orders her to snip off her fingernails on two of her fingers and put the fingers "up my ass". She obeys. They are both delirious with passion.A little later, Rose's funeral and wake are held at the flophouse. Afterwords, Paul sits alone by Rose's coffin. He insults her dead body terribly over why she committed suicide and left him all alone. He then realizes that he never really did know her or the person that she really was. He cries. He calls her "sweetheart". He wished he knew why she killed herself and left behind no note or reason for the act. He wishes he could do the same. He pauses when a prostitute arrives at the flophouse with a client, but Paul chases them away. He follows the prostitute's client a short distance and assaults him before fleeing.A few days later, Jeanne arrives at the apartment only to find it deserted. The "mysterious American" (Paul) is gone. It would appear that he has left her for good. Tearfully, she goes back to Tom and asks him to come look at the apartment. After visiting the flat and looking around, Jeanne asks Tom if it would be a good place for both of them to live after they get married. He doesn't like it and thinks it smells due to a musky odor as if someone had recently had sex in the apartment. He insists that the two of them live somewhere else as adults and not as overgrown, oversexed children. Tom walks out, leaving Jeanne all by herself.A little later, while Jeanne is walking on a boulevard, Paul comes up to her. Jeanne insists that their relationship is over for she has decided to get married to Tom. (She defiantly tells Paul her fiancée's name after he angrily tells her "I said no names!"). Being the egoist that he is, Paul won't accept her decision to break up with him, and he finally begins to tell her all about himself and about how his wife committed suicide weeks earlier (without revealing his name or wife's name). Paul wants to continue their secret relationship of sex to purge his painful feelings about his wife's suicide. All he wants is meaningless sex because he does not want to feel anything at all.Paul and Jeanne spend the rest of the day getting drunk in a cafe where a tango dance contest is taking place. After Paul pesters her, Jeanne alternately tells him that she does love him, but that their relationship is over for she has still decided to marry Tom. They then join the rest of the tango dancers on the dance floor, where she masturbates him, but then runs off, telling Paul "it's finished!" Paul chases after her through the streets, telling her that it will never end.Jeanne runs back to her mother's apartment where Paul forces himself inside and tells her that he will never let her go. Paul tells Jeanne that he needs her in order to continue not to feel anything. It is here that Paul finally asks Jeanne to tell him her name. Just after Jeanne tells Paul her name, he lunges at her and she pulls out a pistol from a desk drawer (her father's military service revolver) and fatally shoots Paul once in the chest. Paul stumbles out to the balcony to view the sights of Paris one last time before he collapses to the floor of the balcony, dead. The final shot shows a shaken Jeanne rehearsing with herself over what to tell the police, as she prepares to call them, telling them that she does not know the man who assaulted her, or his name.