Warning: Wikipedia contains spoilers
William Thacker (Grant) runs a small bookshop in Notting Hill, London. Witty, cultivated and handsome, he has not quite coped with the fact that his wife divorced him, and he is now sharing a flat with an eccentric would-be artist called Spike (Rhys Ifans). One day famous Hollywood actress Anna Scott (Roberts), on a brief trip to London, enters his shop to buy a book. A few minutes later they meet again in the busy street when William inadvertently spills some orange juice down Anna's T-shirt. He offers her his flat as a place to get changed, and Anna accepts.
Right from the start their love affair seems to be doomed. Although Anna feels at home with his circle of friends, William more and more realizes that he is not cut out to be a famous actress's boyfriend. First he is shocked when he finds out that she has an American lover (Alec Baldwin) although Anna asserts that they have broken up. Some time later, after they spent the first night together at William's flat, careless talk by Spike down at the pub results in a throng of reporters at their doorstep trying to take their pictures. She leaves in a hurry, and William decides once and for all to forget her. However, over the months that follow, none of his friends' attempts to find him a girlfriend is successful.
Anna Scott shows up at his bookshop once again, willing to resume their love affair. Only after he has rejected her and she is gone again does William realize that he has just made the biggest mistake in his life. He and his friends search for Anna before she can leave for the United States, and in one of the final scenes, at a press conference, he can persuade her to stay in England with him.