For over 25 years, the BBC gave voice to the silenced people of East Germany by inviting them to secretly write in to a radio programme called Letters without Signature. This documentary explores an unknown story of the Cold War. It looks at the impact of the Letters without Signature series on both the letter writers in East Germany - who faced jail if discovered - and the producers of the show in London, particularly its mysterious presenter, Austin Harrison. Using never-before-seen Stasi files and recordings, London Calling: Cold War Letters documents the tit-for-tat propaganda war between the Stasi and BBC. It reveals a fascinating world of spies, secret state subterfuge and individual acts of bravery.