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 crime epidemic. They find a potential patron in Lord Newcastle, but he is unsure of their plan'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 gangs.

The Fielding brothers are dining at Jack Harris's tavern when a messenger announces that a woman's body has been found at a nearby brothel. Despite horrible injuries, the woman miraculously survives, but the Runners are frustrated by Harris's refusal to hand over his extensive list of prostitutes and their clients. Only through John's guile are they able to obtain it.

Reverend Erasmus Cavendish is found murdered in his Soho church, St. Anne's. His embittered maid informs the Runners that the reverend was a homosexual, a crime still punishable by hanging, and that they should treat a man named William Flynn as the prime suspect. As they search for Flynn, the Runners receive assistance that may not be as clear-cut as it first appears.

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 Fieldings that their prime duty is to protect the property of the rich, and yet burglaries have increased under the police force. Determined to prove him wrong, the brothers search for the suspects in the shanty towns of Covent Garden, called the Seven Dials.

Tom Jones is broken out of jail by his Irish gang, who maim several prison guards along the way. Such a high-profile jailbreak challenges the authority of the Bow Street Runners and risks making Lord Newcastle look very foolish. In a show of strength, the Runners arm themselves heavily to rearrest Jones, with the future of the Runners on the line.