Commissioned to fill a gap and written by David Whitaker in merely two days, "The Edge of Destruction" is really rather excellent, despite its less-than-great reputation in Who fandom. If you're going to do filler do it well.
David Whitaker, one of the most underrated Who writers in any format as far as I'm concerned, has created a completely unique Doctor Who story here. I've seen basically nothing like it done in the programme since and I somehow hope it stays that way. The whole thing is an oddity, the suggestion that the TARDIS is somehow 'alive' even at this early stage in the show, the fact that it is a two-parter in a season filled with long stories, and just the whole atmosphere and feel of the story is entirely different to anything we've seen before, not to mention that the TARDIS is the setting for the entire story and not just for a few scenes.
Basically given the directive 'write an ultra-cheap episode in two days' Whitaker comes up with the idea to turn the Doctor and his companions on each other through an unknown external or internal influence. It's far from a novel or original idea but it is adapted well and differently than you might expect. For a script written in two days Whitaker's work here is solid with most of the dialogue coming off as quite natural.
The acting here is a bit suspect, mainly from Carole Anne Ford as Susan, but mostly solid. The story was only shot on one set so it doesn't look embarrassingly cheap anywhere despite being if I recall correctly the cheapest Doctor Who story of all time.
"The Edge of Destruction" or "Inside the Spaceship" (the latter is probably the more correct title but let's not get into that again) is a nice little 50-minute long diversion and an excellent story overall. I don't really know why so many people hate this one.
Episode One: 8/10, Episode 2: 8/10.
Average: 8/10