Boozy playboy Arthur Bach (Russell Brand) and his chauffeur, Bitterman (Luis Guzman), dress up in Batman and Robin suits for a formal dinner hosted by Arthur's mother, Vivienne (Geraldine James). The dinner is intended to announce Arthur as the new chairman of her corporation, Bach Worldwide. Upon driving to the dinner in a Batmobile, intoxicated and being chased by police, Arthur is arrested and released the next day.



Vivienne and her shrewd assisstant Susan (Jennifer Garner) arrange a plan to have Susan marry Arthur to allow her to be in control of the company, leaving stable leadership. Arthur refuses, but is told that he will be completely cut off from the $950 million inheritance if he does not marry Susan. Arthur reluctantly agrees and asks Susan's father, Burt Johnson (Nick Nolte), for permission to marry his daughter. Burt agrees after putting Arthur to a table saw and warning him not to embarrass Susan. The next day, Arthur meets Naomi (Greta Gerwig), an illegal tour guide who Arthur takes an interest in to due to her rebellious nature. He arranges his wedding while sneaking around on dates with Naomi. Arthur's nanny, Lillian Hobson (Helen Mirren) – who normally dislikes all of Arthur's choices in women – gets to know and like Naomi. Arthur learns that Naomi would like to have her children's book about the Statue of Liberty published. Arthur attempts to find employment and other options so that he would be able to keep seeing Naomi and not need the $950 million, but to no avail.



Arthur goes to see Naomi at her house and tells her the truth – that he is engaged to Susan. Naomi then tells him to leave. Back home, Arthur calls for Hobson, only for Bitterman to come and tell him that she is in bed with a headache. Later Hobson goes to Naomi and asks her to give Arthur another chance, but she again falls ill and is taken to a hospital. Naomi calls Arthur to tell him what has happened. He comes to the hospital, meets Naomi and is about to make up with her but Susan arrives and tells Naomi that Arthur bought the publishing company that was working on her book. Upset, Naomi leaves the hospital. Hobson comes home and Arthur decides to be her caregiver, and the two begin to get along better. But a few days later Hobson dies, and Arthur reverts back to alcoholism and partying to numb the pain of losing her, being dumped by Naomi and getting married to Susan.



At his wedding, Arthur gets drunk and finds Hobson's last letter to him which tells him to follow his heart. Arthur decides, during the vows, not to proceed with the marriage. Susan and her father become belligerent and begin punching him, which makes Vivienne realize that Susan is only after her money. She still wants her to be Arthur's wife, so she warns him that if he pursues Naomi, he will still be cut off. Arthur strips naked to be rid of all dependency on his mother's money, and runs to Naomi's apartment. She is still upset with him and turns down his offer of getting back together.



Six months later, Arthur is now sober. He goes to a bookshop to buy Naomi's book and sees an advertisement saying that she is doing a book reading at a library. Arthur goes to the library, and this time she takes him back. Arthur gets back his inheritance because he did charity work (told people of his adventures). The two leave the library and Bitterman drives them through the city in the Batmobile with the police following in pursuit.