This is another one of my favorite adventures and the placement of this particular two-part episode, being in-between the first Dalek adventure and the first proper historical adventure, couldn't be more appropriate.
In various sci-fi programs that involve travelling to different planets there are usually instances when the action is confined to a starship or the base from which they venture out to different worlds. In the case of this particular program, it is the TARDIS. These are usually referred to as 'bottle' episodes. Making these types of episodes serves a number of unique purposes; it helps cut down on expenses used on developing extravagant set pieces or of going on location for filming and production, it also makes the sense of mystery, dread and impending doom much more focused, thus making the atmosphere quite claustrophobic, and it also provides great opportunities for character development. All of this is achieved to exceptional effect in 'Edge of Destruction'.
In the opening scene we see the crew of the TARDIS knocked out by a mysterious force. Upon regaining consciousness, each of them become dazed and slightly disoriented but they each instinctively know that something isn't right. The fact that so many strange occurrences take place that don't make any logical sense enhances the viewer's lack of knowledge regarding space travel as well as time travel and about the unknown forces that may exist. It also showcases that there is still a great deal we don't know about the TARDIS itself. When you consider how early this is in the show's history as well as in the travels of the main characters, this adventure allows us to share in their confusion and lack of understanding, as they don't know what's going on within the ship any more than we do.
This two-part adventure also showcases the exceptional talents of all four cast members. This is especially true as during the course of the two episodes a line is drawn in the sand between the two alien time travellers and the regular humans.
Carol Ann Ford showcases a great range as she tries to come to grips with what is happening, and in her disorientation becomes highly suspicious of the two people she had held in the same admiration and respect as her own grandfather. The way Susan goes from calm and serene to murderously psychotic and paranoid to utter despair is utterly brilliant.
But the true standouts are definitely William Hartnell and Jacqueline King. The hard felt animosity the Doctor has held for Ian and Barbara reaches its boiling point. During the last two adventures he has had to put up with two irritating strangers that had forced their way onto his ship and had time and again had the utter audacity to try and tell him what to do with his own life. Not to mention having to deal with their constant pestering over things they could never hope to understand, especially considering that he really doesn't feel at all obligated to explain things to people who are far from being his equals on any sort of level. It is this very thought that drives his utter refusal to believe or even consider any of Barbara's theories (at least at first), especially the most important one: that the TARDIS herself was trying to give them vital clue to help them figure a way out of their predicament. This is especially true considering that the Doctor (at this point in time) doesn't want to think that someone of such limited intelligence and comprehension could understand his ship better than he can. This in itself also drives home something that the Doctor doesn't even want to admit; that he doesn't fully understand how his ship works either. Of course, the Doctor being a scientist makes his conclusions and accusations based on hard facts. He accused Ian and Barbara of foul play because to him, it seemed the only logical explanation despite the inherent illogical nature behind the basis of the accusation.
Barbara shows strength of character and obvious venom as she stands up to the indignant and cantankerous old man for daring to accuse her and Ian of trying to cause the Doctor and Susan harm, and of trying to sabotage the ship. She basically showcases that she was not about to be bullied by the likes of him, even going so far as to remind him that it was she and Ian that saved his and Susan's lives (not once, but twice), in addition that it was the Doctor's fault that they were captured by the Daleks in the first place.
The resulting revelation of where the TARDIS had been trying to materialize in (the Big Bang) is very much well written, and the further revelation of the reason why the ship was stuck in the one particular loop of time is both humorous and a bit ironic. Because despite how technologically sophisticated the TARDIS is, the irony comes from the simple fact that something as trivial and seemingly insignificant as a stuck spring could throw all of the systems into disarray.
'Edge of Destruction' also work well as the first of many turning points within the entirety of the series itself. This marks the turning point in which the foursome within the ship go from unwilling travellers and unwanted annoyances to a group of very good friends, almost like a family, with more respect and admiration for one another than ever before. This also marks a turning point in the characterization of the Doctor, especially in the final scene where he goes to make amends with Barbara. It is here that marks the beginning of his change from a grumpy old man to a charming and loveable elderly gentleman that he would be throughout William Hartnell's tenure on the show.
In addition, the line he speaks to her: 'As we learn about each other, so we learn about ourselves' speaks volumes in that the Doctor now has nothing but the highest regard and respect for both her and Ian. It is a bloody shame that their wouldn't be another adventure in this same vein again for this series.