Twenty-five years ago, a stunned world watched as Diana, Princess of Wales poured her heart out on national television. She revealed a royal marriage in crisis, infidelities on both sides and a degree of personal misery that had made her seriously ill. It was watershed television, affecting public perceptions of the monarchy and prompting a constitutional crisis. But what had led to this remarkable moment? In this two-part series we hear from those who witnessed the marriage descend into tragedy and, through previously unseen documents and footage, as well as eyewitness testimony from inside the interview room, discover how and why this extraordinary interview came about.