One day during the run of Michael Frayn's play 'Copenhagen', a curious letter arrived from Celia Rhys-Evans, a housewife in Chiswick. She enclosed a few faded pages of barely legible German, which she thought might be relevant to the mystery at the play's heart. The letter turned out to mark the start of a long and winding trail, as Frayn became first intrigued and then baffled by Celia’s secret.