A series of murder inside a locked room requires the expertise of Henry Merrivale (a fictional amateur sleuth created by John Dickson Carr).
Sir Henry Merrivale is a fictional amateur sleuth created by "Carter Dickson", a pen name of John Dickson Carr (19061977). Also known as "the Old Man," by his initials "H. M." (a pun on "His Majesty"), or "the Maestro", Merrivale appears in 22 of Carr's locked-room mysteries and "impossible crime" novels of the 1930s, 1940s, and 1950s, as well as in two short stories. It contains in chapter 17 the often-reprinted "locked room lecture" in which Dr Fell speaks directly to the reader, setting out the various ways in which murder can be committed in an apparently locked room or otherwise impossible situation. 
