Sherlock and Watson investigate a murder that occurred 20 years earlier. It was witnessed by the victim's son, who claims his father was killed by a monstrous hound. The case takes the pair to the British moors, where a top-secret military base is said to be the site of animal-mutation experiments.
Another great episode from series 2. I like how they adapted and interpreted the Hound of the Baskervilles story. Great suspense and character moments. The "I don't have friends" speech really stands out where we see some of the true madness and genius of Sherlock.