Servers — if one doesn't play, try another
The opener (and only episode, later restored as a short) for an unfinished third series finds a group of detectives assembled in Paris, where they're bumped off in the manner of Agatha Christie's And Then There Were None (aka Ten Little Indians). Copyrighted in 1988, but unreleased until 2014, when it surfaced on The Lost Worlds of Gerry Anderson DVD. It was later included as a bonus feature on the 2017 Dick Spanner DVD set.