The third story of Doctor Who ever made, the Edge of Destruction is one of the most bizarre and surreal stories produced during the Hartnell era, and pales into insignificance compared to the adventures surrounding it (The Daleks and Marco Polo), however one should be more forgiving of its shortcomings, considering that it was made with virtually no budget and was an rushed script by season one story editor David Whitaker. It also delves into the fear and mistrust emanating between the alien Doctor and his granddaughter Susan, and the human school teachers Ian and Barbara, and as such it does add some much needed character development and exposition, as well as proving that the Doctor is an hero with faults, and that he is not always right.
Having said that, the plot for Edge of Destruction is shallow and superfluous and features some incredibly poor acting from the cast (main offender being Carole Ann Ford as Susan).
There are still many things from the plot to me, that doesn't make sense to me, such as the short term memory loss that occurs at the start of the serial, to why an intelligent rational person such as the Doctor could honestly accuse Ian and Barbara of sabotaging the TARDIS without any real tangible shred of evidence ?
It could be argued that the only villain in this serial is the Doctor, he is extremely rude and arrogant to his travelling companions on Earth (in a similar vein to the 6th Doctor to Peri in Twin Dilemma), at one point threatening to eject them out from the TARDIS, and drugging them with sleeping tablets. Susan has some incredibly inexplicable moments, such as the infamous scene with the scissors in episode one. And Ian, normally such an stoic and reliable figure, is prone fits of irrationality. About the only character who remains consistent is Barbara, except for that scene with the melting clock (which again isn't satisfactorily explained) which causes her to go into uncharacteristic hysterics)
The main reason this story fails, is because of the ludicrous explanation at the conclusion, the fast return switch being stuck, is an cop out, and frankly still doesn't explain why the characters (and the TARDIS for that matter) have acted so indifferently throughout this short saga. It would have been a far better resolution to this story if there was indeed an invisible alien presence in the TARDIS, and that the four crew members worked together to flush the entity out of the TARDIS.
The story only serves to alienate the audience away from the main hero (the Doctor) whose brusque and unfair treatment of his companions is unsettling, although it is perhaps redeemed in the ending, where its nice to see the likeable Ian Chesterton being so forgiving of the old time traveler, for all his eccentric ways, and showing the depth of Barbaras hurt and anger at the Doctors earlier behavior was well displayed by Jacqueline Hill, and it is a nice moment when the Doctor finally manages to mumble his way uncomfortably to an apology, and the coldness of Barbara melting in the light of the Doctors sincerity.
At the end of the day though, this serial can only be adjudged as the only real weak link in an otherwise excellent debut season of Doctor Who.
Regrettably, the biggest highlight of Edge of Destruction, comes at the end of the serial with the sole surviving footage of Marco Polo as the cliff hangar to Roof of the World.