73 Yards (Doctor Who)

Soon after landing atop a cliff on the coast of Wales, the Doctor accidentally damages a fairy circle. Ruby finds letters referring to someone called “Mad Jack”, but then she discovers that the Doctor has vanished, and her TARDIS key won't work. Stranger still, an old woman stands in the distance, uttering indistinct words. Making for the nearest village, Ruby realises that the woman is always there, exactly seventy-three yards behind her. Anyone who approaches the woman soon flees from Ruby in terror and loathing, leaving her to embark upon a very long odyssey for which she is almost -- but never quite -- completely alone.