Agatha Christie was one of the most ingenious and prolific mystery writers of all time, and her play “The Mousetrap,” at 1st Stage in McLean, is undoubtedly the most famous of her works, as engaging a thriller now as it was when it first appeared almost 60 years ago. “The Mousetrap” takes place on a wintry night in 1952 at the newly opened guest house of Mollie Ralston (Jennifer Weinreich) and Giles Ralston (John Stange). Married just a year, Mollie and Giles are excited to start their new business even though a blizzard is shutting off the hotel from the outside world.