The UK has a new prime minister. On his first day, Boris Johnson will handwrite four identical letters to the captains of Britain's nuclear submarines. They are one of the most closely guarded secrets of the British government. And those Letters of Last Resort will only be opened if London is wiped out by a nuclear blast. The letters must answer a straightforward but morally complex question: should the UK fire back with nuclear weapons? To deter nuclear attacks, the answer must be yes. But can we stomach that answer, given that nuclear retaliation could endanger the survival of our species? ----   Power Corrupts is written and narrated by Brian Klaas. The executive producer and sound editor is George McDonagh.