In 1887, Chon Lin (Fann Wong) is drinking tea with her father (Kim Chan), the Keeper of the Imperial Seal of China, in the Forbidden City. She tells him her brother, Chon Wang (Jackie Chan), is doing well as a sheriff in Carson City, Nevada, but her father replies that her brother is dead to him. At that moment, Lord Nelson Rathbone (Aidan Gillen), leads a band of Boxers into the city, who attack the Keeper. Despite Lin's efforts, her father is killed and the seal is stolen. With his last breath, he gives Lin a box and tells her to give it to her brother.
Meanwhile, Chon Wang is doing well as sheriff, having captured an impressive array of fugitives. His deputy is relaxing with a book called "Roy O'Bannon Vs. The Mummy", a highly fictionalized account of the events of the first film that now portrays Wang's "Shanghai Kid" as a cowardly sidekick, which Wang is dismayed. Wang receives a parcel, which contains the puzzle box and Lin's letter, telling him their father is dead and that she has tracked the murderer to London.
Chon Wang travels to New York City to find his old partner Roy O'Bannon (Owen Wilson), needing his share of the gold left over from the first film to buy his ticket. Roy has left his brief stint in law enforcement, broken off his romance with Falling Leaves, spent most of the gold investing in the Zeppelin (in truth, he lost all of the gold on his novels), and is now a hotel waiter and part-time gigolo. After they attempt prostitution to pay for a trip to the United Kingdom, the Mayor of New York arrives in search of his daughters, Roy's latest clients. Chon Wang fights off a number of New York police officers, and he and Roy ship themselves to London in a crate. On the way, Wang tells Roy about his father, and Roy swears an oath to help reclaim the seal.
In London, Wang and O'Bannon have their duster and watch stolen, respectively, by a number of street thugs including a young boy named Charlie (Aaron Johnson). After an extensive battle to reclaim these items, they are arrested by the police. In Scotland Yard, Inspector Artie Doyle (Thomas Fisher) thanks the two for defeating the Fleet Street gang and gives Roy his watch that Charlie had stolen. He tells Wang that Lin is also in Scotland Yard, having attempted to kill Lord Rathbone and been dubbed "Looney Lin". Doyle is a reader of the Roy O'Bannon novels, and is enthralled to meet the actual Roy O'Bannon. Roy tries to use this to get Lin released, but it does not work. Meanwhile, Rathbone finds himself not alone in his carriage as Wu Chow (Donnie Yen) sneaks his way on and the two exchange a quick conversation and Rathbone slips him the dagger that killed the Keeper of the Imperial Seal. Sometime later, Roy and Wang wander through London, seeing Buckingham Palace. They encounter Charlie, who lets them into the empty house of a nobleman. Charlie brings it to their attention that the nobleman has an invitation to a gala at the castle.
Roy and Wang arrive that night at the gala, wearing disguises: Roy masquerades as Major General "Sherlock Holmes" (a name he derives from the face of a clock), and Wang is the "Maharaja of Nevada". After turning down the offer to try some spotted dick, Wang and Roy follow Lord Rathbone to a private library. Once they enter, they cannot find him; he has slipped through a secret passage, which Wang discovers in a fireplace. As Roy occupies himself with a copy of the Kama Sutra, Wang enters the secret room, which contains treasures from throughout the British Empire. Rathbone's guards attack Roy, but he is rescued by Lin, who has escaped from Scotland Yard. The three see Rathbone hand the Imperial Seal to Wu Chow, the illegitimate brother of the Emperor of China. Rathbone spots them, and after a brief battle, he sets the barn on fire and escapes, locking the doors behind him to burn them. Lin manages to escape through a hole in the ceiling, while Roy and Wang escape in a car that had been parked in the barn. They tell Artie to tell where Charlie (who has stolen the seal) is. Artie finds out that it is in the wax museum. In the wax museum, they save Charlie from the boxers, but are themselves captured outside by the Scotland Yard. Taken away in a police buggy, they are rescued by Charlie, who later reveals his full name to be Charlie Chaplin. They head to the Royal Jubilee celebration to stop Rathbone and Wu Chow. Rathbone tells them that he is going to kill the royal family and frame Lin. They escape and manage to stop Wu Chow from killing the royal family. Just as he was about to kill Wang, Lin blasts him with a rocket, killing him. Then they follow Rathbone to the top of Big Ben. He throws Roy out of the tower, but he survives when he holds a minute hand. Thinking Roy is dead, Chon fights with Rathbone who had the upper hand. At last, he cuts the rope which was tied to the plank they were standing on, tossing them both outside. Roy saves Wang as Rathbone falls to his death.
Roy and Wang are knighted, as is Artie, whose full name is now Sir Arthur Conan Doyle. Artie decides to become a writer, his stories revolving around his deductive reasoning technique, and asks Roy if he can use the "Sherlock Holmes" name. Roy proposes that he and Wang go to Hollywood to get in on the ground floor with motion pictures. They roll off in a buggy, with Charlie (wearing a fake moustache) stowed away. Wang also manages to open the box his father sent him, finding a message inside reminding him of the importance of family.
What was stolen from Wang and O'Bannon?
duster and watch