The Edge of Destruction is an interesting one. For starters it was written in an incredibly short period of time when they realised they only had eleven episodes for their thirteen episode season, and also because of time and money constraints the two episodes are set entirely in the TARDIS with the four main characters as the only cast.
While doing repairs on the TARDIS, the Doctor causes an explosion and when everyone comes round they’re acting very strangely. Susan becomes convinced that a strange alien force has taken over the ship. The doors are opening and closing themselves and the console electrocutes Barbara as she tries to operate the door controls.
Susan finds a pair of scissors and starts using them as a weapon, slashing at both Ian and Barbara. The Doctor believes that Ian and Barbara have sabotaged the TARDIS in some way to try to return to their home. Their argument is cut short by the realisation that all the clocks are melting. He later brings them a ‘nightcap’ as a peace offering which seems to have actually been drugged. The cliffhanger of episode one is a mysterious someone with their hands around the Doctor’s throat.
Ian stops himself from throttling the Doctor and the women arrive, the four of them all starting to argue amongst themselves once more. The Doctor goes as far as threatening to throw the two humans off the ship before an alert distracts them, showing there are problems in every part of the ship. The Doctor manages to locate the problem to a faulty spring in the fast return switch which is sending them right to the beginning of the universe. All the problems were caused by the TARDIS trying to warn them of the problem before it’s too late.
The Doctor apologises to his companions and they accept, though someone reluctantly in Barbara’s case, as the Doctor’s angry words cut deep.
As much as these episodes are the pinnacle of filler, they are actually quite important. Being entirely set in the TARDIS with the limited cast means there’s a lot of important information in terms of character developments and it gives them all a chance to get to know each other while under a lot of pressure. It’s only after this episode that they truly learn to trust one another.
Also a couple of important things in Doctor Who lore are mentioned, the most prominent being the idea of the TARDIS’ sentience. She does her best to warn her passengers of the impending threat. Ian also exclaims about the TARDIS being alive upon first encountering her in An Unearthly Child but The Edge of Destruction is the first time the Doctor also refers to the TARDIS being alive.
I can’t see anything all that educational in this episode, that being the main theme of early stories. Unless the moral is “Don’t try to attack your teachers with scissors.” Which is important, but hopefully doesn’t need to be said!!