This story was written by then script editor David Whitaker in a weekend at the last minute to fulfill the initial 13 episode order by the BBC. All the other scripts being worked on were longer and wouldn't be given the go-ahead until after the ratings success of The Daleks. So its amazing that this story is so good given all of that.
The surreal atmosphere and the mystery of what is going on is unlike anything else in Doctor Who and it entirely relies on the acting abilities of the main cast to see it through. The story is a bit uneven and the eventual resolution and the leaps of logic needed to get to the solution are a bit much but otherwise its a very solid story script-wise.
This story builds the conflict between the TARDIS crew to a crescendo leading them to suspicion and even suggesting throwing Ian and Barbara off the ship but it also leads to a peace of sorts at the end with Ian and Barbara being accepted as crew members and the Doctor mellowing quite a bit. It is here that he starts moving from anti-hero to the true hero he is later.
William Hartnell, William Russell and Jacqueline Hill are excellent throughout doing an excellent job of building tension and acting oddly in this somewhat surreal story. Carole Ann Ford goes a bit over-the-top at times but otherwise she's fine. This story is another that gives her the opportunity to be a bit more alien.
Definitely a very solid third story. I give it 6 out of 10 TARDISes.