Regnum Defende, or Defend the Realm, reads M.I.5.’s motto. The United Kingdom’s domestic security and counter-intelligence agency, officially named Military Intelligence, Section 5, has existed for over a century. It started out as the country’s Secret Service Bureau, in 1909. During the First World War the organization arrested German spies; during W.W. II it turned foreign moles into double-agents; closer to the new millennium, it investigated I.R.A. associates during the Troubles and thwarted Soviet espionage efforts. Many of the finer details of M.I.5.’s escapades are under wraps—this is a counter-espionage unit, after all. But the National Archives in London has put together an expansive exhibition of formerly top-secret materials from M.I.5.’s entire history. These include original case files, archival photography, and real equipment “used by spies and spy-catchers.” —Jack Sullivan