Informers are playing a key role in helping the Russian government silence dissent, now one victim has turned detective to uncover their persecutor’s true identity. In today’s Russia there is a hunt for the enemy within. Anybody who is accused of voicing opposition to the war in Ukraine can face losing their job, or being prosecuted and facing a fine, or in many cases prison. Academic Aleksandra Arkhipova was denounced by a mysterious serial informer called Anna Korobkova, and decided to become an exile in France. Korobkova boasted of taking pleasure in reporting hundreds of people for allegedly being unpatriotic. But nobody had met her or even knew what she looked like. So, Arkhipova began to piece the clues together to discover the true identity of the anonymous figure who ruined her life. This episode of The Documentary, comes to you from BBC Trending in-depth reporting on the world of social media.