This story has a particular soft spot for me as it was my most memorable early introduction to Doctor who, I was still in single figures regarding my age, but after this one I was hooked for life.
Watching as an adult, of course it has the usual issues with the BBC effects, but remembering that this was 5 years before Star Wars and done on a shoestring budget, it all holds up remarkably well.
Monsters lurking around dark corners, our heroes in plenty of peril, good and evil, lots of literally explosive action and stunts, with many well rounded characters, all filmed with some rather dynamic direction that produces quite a few iconic shots, and add to all of that, some of the most unique use of analogue synthesiser sounds you're likely to come across on any soundtrack.
We also find ourselves travelling over and under the sea in just about every form of water vehicle known to man at the time.
It still makes quite an impression, even today!