This story is set completely in the TARDIS. It’s only 2 parts long, so you can watch it in an hour, which is nice. This is one that I recalled liking as kid, but found kind of disappointing when I watched it again recently.
It is an important story, I suppose, in that it is the first story that introduces the idea that the TARDIS is more than just a mere machine and possesses some intelligence of its own, which oddly, seems to surprise the Doctor. In retrospect this seems weird since many sentient machines from his home world have been introduced over the years (the hand of Omega, Nemesis, the Moment), but at the time this was originally broadcast nobody would’ve questioned it, since the Doctor Who mythos was literally made up as they went along.
There is a moment when Barbara freaks out over what is supposed to be a melting clock (frankly, it’s hard to tell what it is) like it’s the most terrifying of horrors. Don’t ever bring this gal to a Salvador Dali exhibit!
It is also an important story because after saying many horrible things to each other for most of the story, and yes, even trying to kill each other with scissors, The TARDIS Team starts to bond with each other. It’s like a horrible session of family therapy.
After this story, everybody gets along much better more consistently. Hartnell’s Doctor is also a lot nicer after this. After reaching his apex of buttholiness in this story, there was nowhere left to go but down, I suppose. The crew even celebrates their newfound joy at no longer wanting to murder each other by having a snowball fight. Then they discover a mysterious huge footprint in the snow. No, it’s not a Yeti (because that would be cool). You never get to find out what it is in this episode because this is the lead into the next story, which is Marco Polo and is completely missing from the archives. Bummer. This whole missing episode thing is gonna confuse the heck out of casual viewers, don’tcha think?