William "Will" Thacker owns an independent book store, The Travel Book Co. in Notting Hill. He is divorced and shares his house with an eccentric, care-free Welshman named Spike.
Will encounters Hollywood superstar Anna Scott when she enters his shop. Minutes later, they collide in the street and his drink spills on her clothes. Will offers his house nearby for Anna to change. Before leaving, she impulsively kisses him.
Days later, Anna leaves him a message that she is at the Ritz Hotel. Will is mistaken for a journalistâin a panic he claims he works for Horse & Hound, and is made to interview the cast of Anna's new sci-fi film, which he has not seen. Anna calls him back in and says she has cleared her evening for him. Will is exhilarated, before remembering that he is expected at his sister Honey's birthday party; Anna surprises him by saying she will join him.
At the house of Max and Bella, Anna feels at home as she share stories with the small group of birthday party guests. She and Will share a moment in a private garden square. The next day they go to a restaurant, where Will overhears Anna being spoken of in salacious terms and defends her. Anna invites Will to her room at the Ritz, but her American boyfriend, film star Jeff King, has arrived unannounced. Will pretends to be a room-service waiter and Jeff is very dismissive towards him. Anna is apologetic and embarrassed; she thought King had broken up with her. Will realizes he must end things with Anna. Over the next six months, Max and Bella set Will up on a series of blind dates (the last with someone who is "perfect, absolutely perfect"), hoping to help him move on from Anna.
A distraught Anna appears unexpectedly at Will's doorstep; some pre-stardom nude photos have been published, and she needs a place to hide out. Once she calms down, they spend some time on the rooftop patio rehearsing lines from Anna's next film. Back inside, when she sees a poster of the Marc Chagall painting La MariĂŠe, Anna tells Will that "it feels like how love should be". After an amazing day together, Anna goes to him at night and they make love. The next morning, she makes him breakfast in bed and she asks if she can stay. The doorbell rings and they are horrified to find a horde of reporters, alerted by Spike's careless talk at the pub. Spike re-opens the door to be photographed in only his dirty underwear, exacerbating the situation. While Anna gets dressed, she verbally berates Will for what she views as his desire for publicity. He tries to cheer her up, reminding her that the fame is all nonsense, but she tells him the scandal will follow her forever, departing with the statement that she regrets their time together.
Several months and seasons pass. At a dinner with his friends, Will discovers that Anna is back in town to make a Henry James film, which Will had previously suggested to her. He visits her location shoot, where Anna sees him and invites him past security. Given headphones to hear the dialogue, Will leaves when he overhears Anna evading questions from her co-star, stating that Will is no one important.
The next day, Anna comes to the bookshop with a present. Anna admits she still has feelings for him, apologises for her behaviour months ago and explains that she now understands his comment, months earlier, that the fame is nonsense. She says she has to leave tomorrow and is wondering if she did not leave, whether he might let her see him a little, or maybe a lot, whether he could like her again after how badly she treated him. When he brings up the conversation she had with her costar on the set she explains that her costar is a notorious gossiper and she was being discreet. Will turns her down, saying he is unable to handle another inevitable break up, especially considering how big of a star she is and that he would be unable to escape seeing her face if the inevitable breakup happens. He says he is from Notting Hill and she from Beverly Hills. She says with teary eyes, "I'm also just a girl, standing in front of a boy, asking him to love her". She kisses him goodbye and leaves. She leaves Will's present behind, which turns out to be the original La MariĂŠe.
Will meets his friends in a restaurant. They take turns supporting his decision to not see her any more and try to make him feel better. Spike enters the restaurant and is told that Will has decided to leave Anna. Spike calls him a daft prick and lambastes him for turning away the woman he loves. His friends race him across London in Max's car to reach her press conference at the Savoy Hotel. Will adopts his Horse & Hound persona to ask two questions that reveal their relationship and his feelings for her, persuading her to stay in the UK "indefinitely". She breaks into a dazzling smile. His friends in the audience hug and kiss.
A montage shows the two getting married, at one of Anna's movie premieres, then happily sitting on a garden square bench, where Will reads to a heavily pregnant Anna.

Question: Why does Anna tell her costar that Will is no one important?
Answer: Because the costar is a gossip and she was being discreet
Alternative answer: The costar is a gossiper