Tonight's episode reverted back to the stylised episodes of the past where it is character lead with few special effects and lots of good dialog.
Not having a 'physical' creature and depending on the suspense of the unknown entity harks back to some of the classic horror movies.

I feel it paid homage to movies like 'Lifeboat', in that peoples ability to change is exploited, turning mild mannered people into homicidal maniacs.

Great episode, and not a lot of Donna again....