I prefer 1 and 2 for being standalone with fuller case investigation though 3 has its merits it feels to me like we have had one half of the story. The only times a villain has been introduced is when they haven't been shown to be the big baddie - either it's a bait and switch with Mary being introduced as the big bad or we've got half a story. I want to know what the word Guardian means and if it isn't a red herring who she guards is, hopefully, John but I would like to have done with John living away from 221B.
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We solve crimes, I blog about it and he forgets his pants, so I wouldn’t hold out too much hope. (Scandal in Belgravia)
I asked you for one more miracle. I asked you to stop being dead..........I heard you.(The Empty Hearse)