The vast majority of the first series is sadly missing from TV archives and likely lost forever - only the first 15 minutes of this first episode and three complete later episodes, Girl on the Trapeze (1961) (which incidentally does not feature the character of Steed), The Frighteners (1961), and Tunnel of Fear (1961) (found in 2016) have been recovered and preserved.
Whilst the series is forever associated with champagne (normally Pol Roger, Winston Churchill's favourite brand) Steed also drinks whiskey and brandy, normally with soda but without ice. In 'The New Avengers' he drinks beer on one occasion but defends that as an emergency. He does not smoke but carries a cigarette case in case others do (a truism of genuine spies).
This is part one of the only two part episode in the series. This means that even if the missing two thirds of the episode are found in the future, the story will still end unresolved.
Nyree Dawn Porter was the first choice for Catherine Gale, but turned it down. She would later star in a very similar role on 1970s series 'The Protectors'.
The nameplate outside the surgery shows the two partners to be Dr. D[avid]. H. Keel and Dr. R[ichard]. J. Tredding.