Knock knock...who's there....Doctor...Doctor Who, that's who. It's my kids' favorite joke and I'm sure was the beginning of the idea for this episode.
Finally, in a true Who way, we get a story that turns fun into horror then twists into a love story. I think I will cover the plot quickly today then go into some of the questions I was left with. 
We start with the upbeat music and smiling faces of college kids trying to get a place together. At the moment they give up hope, the landlord steps in and presents a perfect ,if not creepy, place to live. The cycle begins again. They are happy and upbeat until the landlord appears again. This time joy is replaced by terror as the walls come alive. The Doctor convinces the landlord that he can help and he and Bill are the last two standing. Once again with Bill's human perspective, the good Doctor figures out the solution and we are left with a life lesson. 
Knock knock was the first episode of this season to approach what I feel is a true Who story. It had twists. It had laughs. And it had the Doctor being a quirky alien. Splitting Bill and the Doctor worked nicely as she could be assertive and the rest of the ensemble could ask the questions. We are left with a lesson about life at the end as well. Very nicely done--the monster became the savior.
Now to the questions that we are left with after watching. The season opened with pictures of Susan and River on his desk. Suddenly, on a whim, Bill refers to him as her grandfather. Add to that the brief conversation about Time Lords and regeneration and Bill is looking like a more important person than just another companion. Could Bill be Susan regenerated? The landlord mentioning leaving your loved ones behind may be a subtle clue to that as well. This season just became interesting.
Next is the vault of course. I have been hoping for John Simm's Master to be locked in a variation of the Pandorica. Possibly Missy helps him escape and we are treated to a "the Two Masters" episode for the season finale. This could still be and I hope it is. The "pop goes the weasel" at the mention of eating children may confirm this too--I hope so. Pardon the pun, but time will tell.
For now, this season has finally piqued my curiosity and I will give "Knock Knock" a fair score with 3 Tardises out of 5.