Secret agent Sidney Reilly, the first "super-spy" of the 20th century, is executed by the OGPU, the secret police of the Soviet Union.

Espionage, spying, or intelligence gathering, as a subfield of the intelligence field, is the act of obtaining secret or confidential information (intelligence). A person who commits espionage on a mission-specific contract is called an espionage agent or spy. A person who commits espionage as a fully employed officer of a government is called an intelligence officer. Any individual or spy ring, in the service of a government, company, criminal organization, or independent operation, can commit espionage. The practice is clandestine, as it is by definition unwelcome. In some circumstances, it may be a legal tool of law enforcement and in others, it may be illegal and punishable by law.

Sidney George Reilly, known as the "Ace of Spies", was a Russian-born adventurer and secret agent employed by Scotland Yard's Special Branch and later by the Foreign Section of the British Secret Service Bureau, the precursor to the modern British Secret Intelligence Service (MI6/SIS). He is alleged to have spied for at least four different great powers, and documentary evidence indicates that he was involved in espionage activities in 1890s London among Russian émigré circles, in Manchuria on the eve of the Russo-Japanese War (1904–05), and in an abortive 1918 coup d'état against Vladimir Lenin's Bolshevik government in Moscow.

The State Political Directorate, abbreviated as GPU, was the secret police of the Russian Soviet Federative Socialist Republic from February 1922 to November 1923. It was the immediate successor of the Cheka, and was replaced by the Joint State Political Directorate (OGPU).

Secret police are police, intelligence, or security agencies that engage in covert operations against a government's political, ideological, or social opponents and dissidents. Secret police organizations are characteristic of authoritarian and totalitarian regimes. They protect the political power of a dictator or regime and often operate outside the law to repress dissidents and weaken political opposition, frequently using violence. They may enjoy legal sanction to hold and charge suspects without ever identifying their organization.