My colleague has already penned his defense of Edge of Destruction, so I felt it incumbent upon me to take up the solemn duty of the prosecution. While he mentions several notable qualities of the episode, I feel there are several flaws that did not receive sufficient attention in his review.
The first consists of the episode’s existence in the first place. When one has a new series, it is inopportune to have a “filler” episode as your third installment. The commentary to the episode describes the circumstances: It was normal practice to make the decision whether or not to continue a series after the thirteenth show of the first season. The producers of Dr. Who felt that their product was of sufficient quality that this review was merely pro forma and that they could expect a continuance even before the thirteenth show. Thus, they planned to have the first two episodes stretch over 11 parts, and the third episode would extend past show #13. Well, that plan turned out to be based on false hopes, so the third episode had to be written in only two parts, and with almost no budget (apparently the money had been blown on The Daleks — this inability to manage the show’s finances will continue to plague the first season).
The second flaw is the way the episode is structured. While watching, the various strange happenings work successfully to create an impression of confusion and inculcate the idea that the world inside the spaceship is off-kilter. However, in hindsight there are a number of significant inconsistencies. Why does Susan go crazy inside her bedroom? Why does she brandish a pair of scissors, and later rip up her bed with them? Why does she threaten Barbara?
Later, the TARDIS apparently melts a large ornamental clock inside the control room, and damages the watches of the crew. How does it do this? Are there microwave emitters throughout the ship, in case the Doctor happens to desire a hot cup of tea and doesn’t wish to walk to the food console? (My colleage also points out that the water dispenser lights work inconsistently in this episode).
Finally, Ian is possessed by the TARDIS, by means unknown, and he attacks the Doctor at the control panel. In order, apparently, to warn the Doctor that the Fast Return switch is stuck. A switch which is on the control panel. Where the Doctor was just looking. The control panel which sent out an electric shock whenever somebody approached it. You’re kind of sending mixed messages, aren’t you, TARDIS old pal?
Nevertheless, the character development in the episode and the fact that this is one of the first true sci-fi mindfuck episodes in television history (if you omit the Twilight Zone) spare this episode from the hoosegow. Verdict: Acquittal (just barely).