When the spooks of MI5 discovered that there had been a British spy in the Atomic Bomb Manhattan Project, at first they couldn't decide if it was Rudolf Peierls – the father of the bomb, who first realised how to build the weapon, or Klaus Fuchs, his colleague, who lived in the Peierls’ household like a son. What information did Fuchs pass to the USSR; what evidence was there; why did he do it and why did he confess? And why, soon after Fuchs’ arrest, did his colleague Bruno Pontecorvo defect to the USSR as the Cold War threatened to turn hot? For the first time a scientist has examined previously unseen documents and unravelled a surprising tale – of love and betrayal, of duplicity, and of questionable behaviour by the Establishment – and discovered that an Abingdon School Music teacher was a mole for MI5. The lecture is based on Frank Close's book “Half Life – Bruno Pontecorvo, physicist or spy”, and his upcoming book “Trinity”.