This episode is essentially a bank heist story on another planet. It has a mystery element and a time travel element. I find it rather quintessential Doctor Who in a number of ways. It has aspects of what makes the show what it is and what the Doctor stands for.
Time travel is used well in this plot without becoming totally illogical. Steven Moffatt's era far too often took time travel aspects too far so that they weren't logical or convincing for me whereas this plot basically makes sense. I love that it makes time travel central to the story because that is part of the essence of Doctor Who - it isn't just space travel it is time travel. The way the mystery is tied in with the time aspect is fun too.
The Doctor using his brains to solve a problem is a feature of this story and that is always a great thing, I think, as that is a key part of the essence of what makes the Doctor a special type of hero. Also the Doctor's compassion in trying to appreciate and help an alien 'monster' is a basic Doctor Who feature which is used well here.
The 'villain' being a banker is a great idea and should be used again in future as banking greed is something which makes perfect sense and relates to real life as well as making sense in alien or future worlds. Keeley Hawes is a very good actress and she plays her part as well as possible adding some class. The other guest acting is all of a good standard as well.
The only real flaw for me is that the supposedly impregnable bank is not all that convincingly portrayed. It hardly seems deserving of its immense reputation.
The adventure is not the most exciting or outstanding one ever but is good solid Doctor Who. The deeper themes of how people behave and how people are treated are thoughtfully presented making this a fairly strong episode overall.
My Rating: 8/10.
My Series 8 Episode Ranking: 6th out of 12.