This is Doctor Who’s first serial of the second season, or serial code L. The previous season ended with his first companion leaving for a less adventurous life, and The rescue is a two episode serial introducing the new companion Vicky. In that sense it is important in the Doctor Who history: here the BBC established the Doctor can stay the same, but his companions might change over time. This was before the actual first regeneration of the Doctor himself. If you don’t know what I’m talking about, I suggest you read my piece about the Tenth Planet.
This short story seems to tell about just another monster of the week, but then turns into a whodunnit when it turns out the monster is not really a monster. The main characters are hiding something horrendous that happened in the past.
Entertaining is it might be, it also very dated. The ways of the Doctor from the 60s are fun from a historical perspective, but it is a bit like watching James Bond and trying not to cringe. Now that I have listened to the excellent portrayal of David Tennant not so long ago, this is a bit… challenging.