It is Boxing Day circa 1935. The place is a snowed-in manor on the very edge of Dartmoor. It is a Christmas house-party. And overhead, in the attic, the dead body of Raymond Gentry, gossip columnist and blackmailer, shot through the heart. But the attic door is locked from the inside, its sole window is traversed by thick iron bars and, naturally, there is no sign of a murderer or a murder weapon. Fortunately (though, for the murderer, unfortunately), one of the guests is the formidable Evadne Mount, the bestselling author of countless classic whodunits. In fact, were she not its presiding sleuth, "The Act of Roger Murgatroyd" is exactly the type of whodunit she herself might have written.