It's the 1930's. American sisters Eugenia Crocker (Allison Janney) and Nesta Pett (Brenda Blethyn) are extremely wealthy and extremely competitive, with each disliking the other. Their latest quest of one-Upmanship has Nesta trying to marry off her niece by marriage, poetess turned crime novelist Ann Chester (Frances O'Connor), to Lord Reginald "Wizzy" Wisbeach (Kevin Eldon), so that there will royalty in the family. This move is against Ann's wishes as she doesn't love the stuffy Lord. Nesta lives in NYC but has come to London with Ann to help arrange the match.Meanwhile, Eugenia, now living in London & married to Bingley (Tom Wilkinson), is trying to buy a royal title. She is trying to foster closeness to the royalty (Duchess of Arminster (Delia Lindsay), Percy (James Wallace), Duke of Arminster) by inviting them to her mansion. Eugenia's current quest and others like it are always hindered by the notoriety of her stepson, James Crocker (Sam Rockwell) - better known as Piccadilly Jim, for the newspaper gossip column he used to write and the job from which he got fired - who is known as a womanizer, brawler, gambler and drunk. Jim had an earlier scuffle with Percy and when he arrives at the mansion, Jim decks him again, to the horror of his mother Eugenia.Nesta visits Eugenia to mock her for Jim's antics. Eugenia retorts that Jim is best friends with Wizzy, whom Nesta is pursuing for Ann. Nesta had assumed Bingley to be the house butler and Bingley left before Eugenia arrived to host Nesta.Jim is thinking about becoming more respectable when he meets and falls in love at first sight with a beautiful American visiting London. That woman is Ann, who hates what she knows of Jim, not only for that notoriety, but also because Piccadilly Jim the columnist once criticized her book of love poems (although it was not actually Jim who was writing the column at the time) which she felt ruined her career.Although Jim doesn't know why Ann doesn't like "Piccadilly Jim" as she has never met him, he assumes the name Algernon Bayliss, who is actually the Crocker family butler. Jim meets Ann at a nightclub but says he doesn't drink. Jim has confided to Bingley that he has assumed Bayliss's identity to get close to Ann. Once Ann leaves, Jim again gets into a scuffle with Percy and decks him, photos of which are splashed all over the newspapers the next day. Jim decides to pursue Ann in NYC. Bayliss (Geoffrey Palmer), the real butler comes to see off Jim at the harbor and Ann assumes that Bayliss is Jim's father (given Jim had given his own name as Bayliss).Ann is not crazy about love, a feeling that Jim tries to sell her on. Ann, who wants a man with an adventurous streak, too falls in love with "Bayliss", despite his overwhelming respectability, although she would prefer if he was a little more dangerous. Ann tells her uncle Peter Pett (Austin Pendleton) about meeting Bayliss in London. Meanwhile a man approaches Nesta in NYC, introducing himself as Wizzy (Hugh Bonneville). The man inquires Nesta about her nephew Willie Partridge (Tom Hollander), who is working on a chemical called as Nitroglycerin.Meanwhile Ogden (Rupert Simonian) Peter and Nesta's son is a spoiled brat and has falsely accused Peter of hitting him when alone. Ann discusses the matter with Jim, who suggests that they should kidnap Ogden and send him to a boot camp to drill some manners into him. Ann suggests that they conduct the kidnapping in the name of Piccadilly Jim. Since Peter and Nesta have never met Jimmy, Ann asks "Bayliss" to pose as Jim. With this, Jim has the license to act himself, drunk, arrogant and a complete scumbag.Jim is astounded to see Bingley in the Nesta house, playing the butler Skinner (apparently Nesta had told him to look her up, if he ever was in NYC and wanted a job). Nesta tells Jim that his close pal Wizzy is staying with them. But when Jim meets the guy, it's not Wizzy at all, but Jim doesn't say anything.After lunch Wizzy introduces himself as a German spy "The Aristocrat" and agrees to keep quiet about Jim if Jim also keeps his secret. The Aristocrat then goes to Willie and plants a doubt that Jim doesn't look like Jim and might be a spy after his formula. He voices the same fears to Nesta, who hires a private detective to keep an eye on things. In Ann's diary, Jim finds the Piccadilly Jim article, in which he had thrashed her poetry, even though he was not employed by the magazine at the time, and yet they published articles under his name. Ann is distraught and tells Jim to give up the plan as she has decided to marry Wizzy. Jim tells her that Wizzy is an impostor but won't believe Jim.To prove his love to Ann, Jim kidnaps Ogden with Bingley. Jim then catches The Aristocrat stealing Willie's explosive sample from Nesta's safe. Aristocrat cries for help, and Willie intervenes. Meanwhile the detective confronts Bingley and Ogden and brings them back into the house. Aristocrat is exposed when the explosive sample is found on his person. Jim rescues Ann and kisses her. Peter puts his foot down and tells Nesta to send Ogden to a good school to teach him some manners. Bingley reveals himself as Eugenia's husband and introduced Jim as his son. Ann is heartbroken but forgives him as he is the adventurous man she was looking for. Ann and Jim marry. Eugenia is granted the royal title and moves to NYC, 3 doors away from Nesta.