It's London, 1748, and James Macleane (JONNY LEE MILLER) has been sent to debtors' jail until he's sober and able to pay his debts. His lucky break comes when a team of outlaws, led by Plunkett (ROBERT CARLYLE) literally crashes into his cell while attempting to rob a coach. Plunkett's partner is killed, but not before he swallows a valuable ruby, and both Macleane and Plunkett escape, but go their separate ways.
They're reunited, however, when both arrive in a cemetery to dig up that ruby and are arrested and sent to prison. Although they don't get along -- Macleane is more of a gentleman while Plunkett is an unkempt scoundrel -- they make a pact to get out by bribing the prison officials with the ruby.
Once free and desirous to head to America, but currently lacking the funding, the two then set out to form a partnership utilizing their best qualities. Macleane will hobnob with the aristocrats through his foppish friend, Lord Rochester (ALAN CUMMING), and find out the travel plans of the rich so that he and Plunkett can then rob them.
Their first catch is a coach carrying Chief Justice Lord Gibson (MICHAEL GAMBON) and his ravishing niece, Lady Rebecca (LIV TYLER). Since they're masked and nonviolent, with Macleane being particularly nice to Rebecca since he's previously fallen for her, the two become known as the Gentlemen bandits.
Nonetheless, Gibson wants them caught and puts the pressure on Chance (KEN STOTT), a determined, stodgy and humorless officer, to put an end to what's becoming a popular criminal spree. Yet, while posing as a gentleman and his servant, Macleane and Plunkett also become popular among the elite when the latter stands up to Gibson who doesn't yet realize they're the criminals he's after.
As their robberies continue and they become increasingly popular in both their civil and criminal personas, the two men begin to split over their ultimate goals. With Macleane enjoying the limelight and Plunkett only wanting to get enough money to head for America, the future of their ways soon comes into doubt.