Jeremy Brett as Sherlock Holmes had by this stage in the series long inhabited the character quite faithfully. In this episode some of his exchanges with Watson(the ever dependable Edward Hardwicke) at their lodgings in the village near Sir Robert's sister property are infused with more dark humour than usual allowing even a degree of character development when such a feat would have seem beyond possibility (or necessity).
The scenes with the spaniel in the ruined chapel have a victorian gothic, almost macabre ressonance contributing to a highly effective and atmospheric adaptation.
The plot is ingenious if a little contrived towards the end.
This episode also marks the tv debut of a very young british actor, but his role is a small one and I feel certain that nobody would guess what kinds of roles he would go on to play just a few years down the line. But perhaps that is best left for the trivia section...