Classic Who: The Tomb of the Cybermen (Season 5, 1967)
Details
In a time before home video, the BBC would routinely destroy tapes and film in their archives to make room for new stuff. As a result, more than 100 episodes from the first 6 years of Doctor Who - particularly the Troughton era - are lost or missing.
Since the advent of video tapes, many single "orphan" episodes have been found. However, it wasn't until 1992 that all 4 episodes of "The Tomb of the Cybermen" were discovered in Hong Kong, making it the earliest complete story from Troughton's TARDIS tenure.
Alongside Troughton - a big inspiration for 11th Doctor, Matt Smith - are companions Jamie McCrimmon (Frazer Hines), the "hairy-legged Highlander" from 1746, and Victoria Waterfield (Deborah Watling), the aptly named Dickensian waif.
[TRIVIA: Jamie traveled with the Doctor for a whopping 113 episodes from 1966-1969, making him the longest-serving companion in Doctor Who history.]
The Discontinuity Guide calls Tomb "well-directed" and "expensive-looking."
Charlie Jane Anders of io9 cites the second episode cliffhanger as one of the greatest in the history of the series.
(It also contains one of the most iconic pieces of music from the early days of Who, and a personal fav.)
Looking forward to watching this with y'all!