It's 1887 and Chon Wang (JACKIE CHAN) is the sheriff in Carson City, Nevada who can't believe that the exploits of his former associate, Roy O'Bannon (OWEN WILSON), have now been put into print. Yet, Wang soon has more to worry about when he gets word that his father has been murdered, the Imperial Seal his family had been guarding was stolen, and that his sister, Chon Lin (FANN WONG), has followed the murderers to London.
Accordingly, Wang heads for New York to find Roy and reclaim his half of the money Roy was supposed to invest for them. Not surprisingly, he's lost that and is in trouble with the law again. After eluding them, Wang and Roy then stow away in a cargo hold to get to London. Once there, and following a run-in with Charlie (AARON JOHNSON), a larcenous street urchin, as well as a street gang, they meet Artie Doyle (THOMAS FISHER) of Scotland Yard.
He's happy that they helped nab that gang, but is unhappy to report that Lin has been jailed due to reportedly trying to kill Lord Rathbone (AIDAN GILLEN), the Queen's cousin and tenth in line for the throne. It turns out he's in cahoots with Wu Chan (DONNIE YEN) who wants to use the Imperial Seal to gain control of China all while helping Rathbone eliminate those ahead of him on the power list.
From that point on, and as Roy becomes sweet on Lin, the three of them, with help from Artie and Charlie, set out to prove the conspiracy and avenge the murder of Wang and Lin's father.