The circumstances surrounding The Edge of Destruction's production should make it immune to criticism. There was a two-episode gap in the schedule, there was no budget and no time, just four regulars and the standing sets in the TARDIS. It was this or nothing.
No, not even a monster, so the menace is intangible and psychological. Whether by design or it's just me, the plot is baffling - it centres on the TARDIS trying to warn everybody that they're on the brink of disaster because a spring is stuck on the fast return switch (a switch that must have been invented for this story as it would have been very useful when they were leaving 100,000bc), giving them bonkers clues (a clock melting?) and apparently making them act weird.
Unfortunately, the story is an exercise in failure across the board. When the TARDIS goes wrong, the characters suspect each other of sabotage, act randomly threatening and collapse for no reason. Most of it seems more like bad acting and left me scratching my head as I tried to figure it all out. I was really quite embarrassed for the actors at times. Aside from one striking scene of Susan threatening Ian with scissors, none of the out-of-character moments are worth the inadequate explanation - the TARDIS did it - and disappoint. One such moment is a cliffhanger!
The story improves in the second episode. Putting aside Barbara inexpicably figuring out clues that make no sense even after they're explained, as soon as the Doctor realises the spring is stuck and they all have a good laugh about it, the nonsense is over, not least because writer David Whitaker doesn't have to pull a story out of nowhere anymore. The Doctor's excited speech about the formation of a solar system is a story highlight, with William Hartnell selling the Doctor's childish glee, and the aftermath where the Doctor realises he's been foolish to mistrust Ian and Barbara leads to a nice scene where the Doctor makes his first proper effort to bond with Barbara, praising her for her ingenuity, followed by an amicable exchange in the control room where the TARDIS lands.
During the first episode, one would wonder if they're watching the same show as the one that just gave us the Daleks, but by the end of Episode 2 I was keen to carry on immediately with the next story, and that's quite a recovery which is all down to the last few minutes, as the Doctor is no longer hostile and selfish, and the TARDIS crew are now a proper team, with the Doctor and Ian even sharing a joke as they leave the TARDIS. This is what we've been building to since the first episode of the series.
However, that stuff only takes up five minutes of the story. The other 45 minutes seems like improvisation or rehearsal. I often read the classic series criticised for its low budget, and the excuse is usually that it wasn't expected to still be watched 40 years later. The Edge of Destruction sees the budget plummet as far as it will go and the writer accordingly coming up with as minimal a story as possible. However even this seems to have been too complex a story to manage with no budget. I can see the writing, performances and direction are all aiming for a psychological horror story, but all departments fall flat on their faces. I wonder if this last-minute necessity for a two-parter had the people behind it gritting their teeth until it was over, reassuring themselves that at least it was only two episodes and it would be quickly forgotten. Hah!
Horror quotient - Susan's scissors scene is the only horror element that works as intended. Part of the reason why is that it's no sci-fi danger, it's something you can find in your kitchen.
Comedy quotient - Loved the Doctor's humbled behaviour at the end. He's in the wrong and he knows it, but his reaction goes against the "grumpy old man" label the First Doctor so often gets.
Drama quotient - I was disappointed. There were dramatic moments but the resolution was inadequately explained and made it all seem pointless.
A couple of scenes aside, a forgettable misfire.