This is a major favorite of mine.
While re-listening to it today, I was struck by a curious thought. It was produced in 1977. It's set in the year 1998, which was 21 years in the future for the show's original audience. They had a 'cushion' of two decades between the events of the episode and their own present-day. For today's audience, 1998 is just a few years ago. Having this episode set in our recent past made it seemed more immediate to me, more suspenseful. The 1990's setting, which would have seemed 'unreal' to the 1977 audience, is more 'real' to today's fans than a show set in the 1970's. It is for me, anyway, and I do remember 1977.

We're living in the future.