This story is rather uneven. On the one hand are some rather odd acting moments, e.g. Ians false strangulation gestures and Susans very moody stares, but on the other the audience sees some great confrontations, particularly between Barbara and the Doctor near the end of episode one. The Doctors verbal attack is strong enough to make the audience fully support Barbaras reaction and she stands up for herself wonderfully. Hartnell also has his best episode yet in The Brink of Disaster he transforms from a hostile accuser to an apologising kind man who has learnt something very important. His flustered inability to apologise to Ian is great to watch, as is his decent apology to Barbara at the end. No other episode in the history of the series ever had the Doctor learn so much about friendship and himself.
All of this drama is enacted within the TARDIS and involves no one else, giving us time to see more of the TARDIS interior and the dynamic of the crew. I feel the direction isn't quite good enough to make the audience believe there is an intruder aboard, and the initial lapse of memory results in some odd moments between Ian and Barbara at the beginning. Susans behaviour is particularly worrying she certainly looks possessed but when the cause of the crisis is revealed, this is shown to be false, so her violent scissor attack was based on paranoia, fear and hysteria rather than possession. Freud would probably have liked to ask her some questions after that one.
The storys main boost in my opinion is not the Fast Return switch problem but Hartnells performance. He is suspicious, worries, cares, bullies, learns, speculates and apologises all in a few scenes in these episodes. When he fails to bully Barbara he offers the crew drinks, his explicit intention to calm everyone down hiding his real intention of forcing them to sleep so he can solve the mystery himself. The scene where he checks Ian and Barbara, chuckling to himself, is wonderful, for he is childishly enjoying himself. The next episode forces him to admit he needs the help of others to solve the problem (a nice touch for from Pertwee onwards he becomes so clever and invulnerable that he rarely needs anyone to help him). However his potential to commit murder is raised again (not seen since the Za stone incident) but this time its worse for he is going to force two acquaintances out of his TARDIS who have previously saved his life and are the victims of his prejudiced false conclusions of events he has shown he doesn't understand. This act makes his desire to ask Barbara to forgive him at the end more touching not only has he learnt an important lesson about himself, he needs forgiveness to move on. His line You still havent forgiven me have you? is very poignant and when Barbara does forgive him the audience knows this crew is now much stronger and warmer than it has ever been before. It is a significant development and a sign of David Whitakers writing skills as well as the acting of the regular cast.
The stock music adds to the drama very well, but I associate it more strongly with The Moonbase and kept thinking of that story when I heard it. It makes the mystery stranger. The cause of the problem is almost incidental to the suspicions it has raised, but the final moments when the Doctor fixes the spring have sufficient drama to make us urge him to hurry up before theyre all annihilated. The final moments are nice to see the tension has gone once the Doctor is forgiven and they are all looking forward to exploring their next destination. The audience will see a kinder Doctor from now on, though his complex fascinating nature will still remain