In the lawless London of the 1750s, Henry Fielding, the humane novelist and magistrate, and his blind brother John, set up the Bow Street Runners, an embryonic police force with three other men who must be above corruption. They are bank-rolled by Lord Newcastle. Whilst dining at an inn run by pimp Jack Harris, word comes to the Fieldings that a prostitute called Ann Bell has been raped and half-killed at a nearby bath-house. It is vital that the criminal be found, not solely in the interests of justice but to prove that the Runners are worthwhile.

Investigating the murder of the Reverend Erasmus Cavendish leads the Runners to a 'Molly house', a brothel and social club for gay men, especially those who like to dress as women. Young Thomas Deacon, also known as Miss Kitten and recently half of a gay 'wedding' is implicated as committing murder in order to prevent his secret being exposed to his employers. There is a conflict of interests as one of the Runners is himself a Molly and involved with Miss Kitten.

A break-in at a sugar merchant's Mayfair home brings the Bow Street Runners into conflict with their patron, Lord Newcastle. He reminds the Fielding brothers that their prime duty is to protect the property of the rich and yet the number of burglaries has increased under their new police force. Determined to prove him wrong, the brothers search for the Irish suspects in the shanty towns of Covent Garden, known as the Seven Dials.

Tom Jones, an infamous gang leader, is broken out of jail by his Irish gang, who maim several prison guards along the way. This high-profile jailbreak questions the authority of the Bow Street Runners and risks making Lord Newcastle look very foolish for supporting them. The Runners travel to the Seven Dials to re-arrest Jones. With Lord Newcastle reviewing the Runners' government allowance, Henry Fielding accompanies them to make sure Jones is apprehended.

Henry looks back on the struggle to fund the Bow Street Runners in the first place, and reflects on how close London came to not having a police force at all. As the new magistrates of Westminster, novelist Henry Fielding and his blind half-brother John want to set up a police force to tackle London's soaring crime epidemic. They find a potential patron in Lord Newcastle, the Prime Minister's brother, but he is unsure of such a force's merits. With the help of Saunders Welch, the High Constable of Holborn, the brothers set out to show Newcastle they can rid the streets of the violent criminal gangs.