A rationalist-for-hire – he debunked apparently supernatural events – is found murdered. Sherlock and Watson discover that he’d been hired by a woman to look into the case of her mother, who claimed that the ghost of her husband was haunting her house, shaking the walls, calling her name, and so on. She even has a recording of the ghost’s voice. On investigation, it turns out that the vibrations were caused by tunnelling under the house, which has exposed a key transatlantic internet cable. And Sherlock discerns, from listening to the recording, that the voice is actually a man shouting in Arabic. It has to be terrorists; except, of course, it isn’t.
The B-plot is actually quite interesting: Gregson’s daughter Hannah is back, and looking for some help from Watson in respect of three drugstore robberies in her neighbourhood. Hannah’s motives turn out to be mixed, though, and the storyline throws up the possibility that she might be less than honourable. And perhaps not that good a detective either. It all adds up to a generally enjoyable episode, if somewhat forgettable; it’s less than 24 hours since I watched it, and the desert sands of my mind are already starting to close over it.