Arthur, portrayed by Russell Brand, is a carefree playboy with a penchant for revelry and an affinity for excess drinking. During a chaotic evening heading to a dress party, he and his loyal but quirky sidekick Bitterman, played by Luis Guzman, don Batman and Robin costumes, leading to their unexpected arrest. However, true to his extravagant nature, Arthur secures their release from jail.
The following morning, the stern yet nurturing Hobson, brought to life by the talented Helen Mirren, undertakes the task of tidying up Arthur's mess. With the heart of a guardian, she attempts to guide him towards sobriety, insisting he accompany her to meet his mother, Vivienne, portrayed by Geraldine James. Vivienne reveals the family’s new expectation: Arthur must marry Susan Johnson, played by Jennifer Garner, in order to maintain control over their family business.
Despite his mother’s demands, Arthur's carefree spirit leads him to spend money recklessly. An encounter with the free-spirited Naomi, brought to the screen by Greta Gerwig, ignites sparks between them. Arthur is captivated, expressing his desire to see her again, but the shadow of marriage to Susan looms over him.
Arthur seeks permission from Susan's father, Burt, portrayed by Nick Nolte, to marry her. Burt's approval stems from a desire to see Susan ‘straighten’ Arthur out. During a dinner date, Susan begins to take control, but Arthur resists her manipulative tactics. In a bold move, he commandeers Grand Central Station for a dinner with Naomi, who is charmed by his wealth yet yearns for genuine connection. She invites him to her humble abode, where they bond over a meal of canned pasta, and Arthur encourages her to pursue her writing ambitions.
As their relationship blossoms, Susan becomes increasingly threatened and sabotages Arthur and Naomi's bond. Meanwhile, Arthur decides it’s time to take responsibility and attends an AA meeting, only to be embarrassed when Hobson shares about his life.
With mounting pressure, Arthur prepares for his wedding while grappling with his hidden misery. Hobson, concerned for Arthur's happiness, appeals to Vivienne and later implores Naomi to recognize Arthur’s love for her. Unfortunately, Hobson falls ill, prompting Arthur and Naomi to worry for her well-being. In a cruel twist, Susan confronts Naomi, falsely accusing Arthur of coercing her.
Arthur dedicates himself to caring for Hobson, starting with simple meals of canned pasta but gradually improving her diet. He learns of Hobson's sacrifices made for him, which leads to her heart-wrenching passing, leaving Arthur devastated and spiraling back into alcoholism.
On the day of his wedding, Arthur discovers a heartfelt note from Hobson urging him to pursue Naomi. Despite Susan's desperate attempts to assert control over him and the family company, Arthur defiantly rejects the marriage. In a dramatic gesture, he strips down and rushes to find Naomi, but she rebuffs him. Seeking solace, he visits a church, resolves to stop drinking, and discovers Naomi's new book, which is dedicated to him.
Summoning the courage to apologize, Arthur finds Naomi, and the two share a kiss as he reveals his mother has forgiven him. They leave together in a nostalgic nod to their adventures — in the Batmobile, ready to embrace a new chapter together.
