After a British cabinet minister is assassinated in London, an undercover operative is dispatched into the volatile streets of Belfast to track down the killer. Harry Brown, an agent lacking sufficient preparation, must navigate the hostile environment of Northern Ireland to infiltrate an IRA cell. He faces the constant threat of discovery in a city where every stranger is scrutinized, forcing him to rely on his wits to survive. The narrative follows his descent into a high-stakes game of cat and mouse, where the pressure of the mission and the isolation of his position create a precarious situation.

This work is widely regarded as a seminal entry in the modern espionage genre, noted for its stark realism and lack of romanticized heroism. Critics often highlight the author's ability to capture the grim atmosphere of the Troubles with clinical precision and narrative urgency. The story is frequently cited for its influence on the development of the contemporary political thriller, particularly in its focus on the vulnerability of the individual agent. Readers typically respond to the relentless tension and the author's refusal to provide easy resolutions to the complex conflict presented.