Arthur (Russell Brand), the heir of the wealthy Buck family, is a well-known Peter Pan-like figure in New York City. His absurd behaviors such as dressing up as Batman and running rampant are often reported in the newspapers, which makes his mother, who is in charge of the family company, feel pressured. Arthur's behavior like an ignorant child often requires the old butler Hobson (Helen Mirren) to clean up the mess. His mother can't stand Arthur's behavior of smearing the company's reputation, and threatens Arthur to marry his capable female assistant Susan by cutting off his financial resources. Susan is quite ambitious in her career and is determined to rely on this marriage to improve her status as a builder. Seeing that Arthur bowed to his mother for money, he met Naomi (Greta Gerwig), an unlicensed tour guide, on the street. The latter's style made Arthur fall in love. At the crossroads of marriage and love, Arthur hopes to find a perfect ending, which may be the strongest challenge he has ever faced in his life.