Night Terrors.
We're sometimes a bit hard on Mark Gatiss, but by golly what a cracker of an episode. No River, no arc-y faff, just a gorgeous, small-scale, scary & funny & surprising & intriguing episode - and for yer fanboy, its little nods to Paradise Towers and the Celestial Toymaker only add to an already full mix. Even the direction is top-notch.