Through the years playgoers and critics joined in keeping any secret Mrs Christie confided to them, and her trust was honoured; it astonishes us now that after a quarter of a century in London The Mousetrap can still be acted before audiences with no idea of its development or climax.

Agatha Christie, by herself, wrote twelve full-scale plays (one published, not performed) and three in a single act. She collaborated in another full-length play; four more, from her novels or short stories, were adapted by other hands…. Whatever else was wrong, nobody sustained a problem as she did, or solved it so quickly without a tedious explanatory huddle. This was her Midas gift to the theatre. 'Upon my soul,' exclaimed [Charles] Dickens's Barnacle Junior, 'you mustn't come into the place saying you want to know, you know.' Agatha Christie's fans did want to know. In the...