It's the very first Doctor Who story, and a lot of what the nearly 60 year old franchise is, owes itself to An Unearthly Child. A lot of it was developed right here, right out of the gate. A lot of the potential of the series was ready to go, here. The Doctor's character is mysterious and I feel he is well introduced through the eyes of Ian and Barbara. Them piecing together the strangeness of Susan, them discovering the Doctor and the TARDIS for the first time, William Hartnell's borderline sinister acting early on giving a sense of mysteriousness and intrigue, all of it serves to create a fine series I am glad to be a fan of. A lot of this holds up even if yes, most of it is thanks to all the steam and energy put forth by the first episode. The rest leaves a bit to be desired, but it is still absolutely a must-watch for any big fan of Doctor Who.