After a trip to the future and a quick jaunt to the past, it's now time for Doctor Who to do the other thing that it's managed so well at in the past - good old fashioned present day horror. The fourth episode in series 10 of the modern era is Knock Knock - written by series newcomer Mike Bartlett and guest starring the legendary David Suchet , perhaps most famously known for his role as Hercule Poirot in Agatha Christie's Poirot. Following the episode's original airing a special "surround sound" edition was also released by the BBC , giving the viewer even more immersion into it's creaky wooden house setting.