Really, really good, but I am a bit put off by some plot details. In spite of excellent atmosphere and some strong moments, the ending felt a little rushed and unsatisfying. Midnight is a good example of the mystery feeding into the horror, but here, I don’t know, I think we are ought to have a bit better of an explanation.

Here’s my big question I am really struggling with since it is a central point of the episode.  Why did future Ruby scare away anyone who encountered her?  It seems like such an essential part of the episode and we are offered no sense of why. Midnight gives us at least a sense of there being rules to its monster even if we never quite learn its nature. Here, I’m not entirely sure what exactly was going on. It’s not a big deal because it’s a good episode overall. Really strong cinematography, direction, and production that reminded me of a top-tier modern horror like It Follows or Talk to Me.  In spite of my problems it is a cool episode and great creepy fiction even if the plot was a bit jank.  It's a really cool episode for Ruby, too, nicely focusing on her and totally selling me on the character.

EDIT:  Honestly, the more time has passed, the less I care about this story not explaining everything.  The Empire of Death drops a hint that 73 Yards is a relevant measurement to the TARDIS and it made me really appreciate this episode a little more.  Because you can have these little hints and intricacies that only add more questions and intrigue to the story.  I really like 73 Yards now.  I think it can go down in history as one of the best of Doctor Who.