A newly published book offers an extensive look into the inner workings of MI5, Britain’s domestic intelligence agency, revealing decades of espionage, counter-terrorism efforts, and clandestine operations. Drawing from some 6,000 files declassified since 1997, the volume provides insights into the Security Service’s activities from its early formation in the early 20th century to more recent threats.

The book presents a comprehensive account of MI5’s role in key historical episodes, starting with the hunt for German spies during World War I. It details the agency’s involvement in countering British fascism in the interwar years, focusing on figures such as Oswald Mosley. Among the revelations are previously unpublished images and files related to well-known espionage scandals, including the Profumo Affair of 1961, where then-Secretary of State for War John Profumo engaged in a clandestine relationship with model Christine Keeler, who was linked to a Soviet attaché.

Significant attention is devoted to the Cambridge Spy Ring, made up of Kim Philby, Guy Burgess, Anthony Blunt, Donald Maclean, and John Cairncross. These individuals passed sensitive information to the Soviet Union for decades. Newly disclosed documents illustrate the intense efforts by MI5 to identify and monitor these double agents, revealing the anxiety their covert activities generated within the British establishment.

The book also covers women’s critical contributions to MI5 operations, including Olga Gray’s work infiltrating a spy ring in 1938 and longstanding recognition, highlighted by MI5 spymaster Maxwell Knight’s 1945 observation that many of the greatest intelligence coups were achieved by female agents. Other intriguing anecdotes include the capture of German spy Karl Muller in World War I, whose use of lemon juice for writing invisible ink ultimately led to his arrest.

Moving to more contemporary challenges, the authors examine the 1991 IRA mortar attack on Downing Street, an incident that exposed vulnerabilities in British counter-terrorism before the Security Service assumed a primary role in monitoring IRA activities domestically. The subsequent appointment of Stella Rimington as the first female Director General of MI5 in 1992 attracted considerable media attention, reflecting changing perceptions about the agency.

One of the largest recent operations detailed is Operation Overt, a 2006 joint MI5 and Metropolitan Police investigation that thwarted a plot to bomb seven flights departing from Heathrow Airport using explosives concealed in soft drink bottles. The operation succeeded in arresting the plotters, including Abdulla Ahmed Ali, before the attacks could be carried out. Since then, MI5 and police have disrupted more than 40 major terrorist plots, although officials acknowledge the ongoing difficulty in preventing every attack.

The authors, archivists Mark Dunton and Stephen Twigge, offer a blend of historical analysis and anecdotal detail, supported by photographs and original documents from MI5’s extensive archives. The foreword, contributed by author and intelligence expert Nick Harkaway, emphasizes the enduring significance of secrets in shaping political and security events and underscores the high stakes faced by intelligence officers operating in the shadows.