The opening credits include scenes from that week's episode. Gerry Anderson previously used this gimmick in "Thunderbirds" (1964) and "Space: 1999" (1975).
As well as a "This Episode" segment, there was originally supposed to be been a "Next Episode" segment as well, just prior to the closing credits. A piece of music was specifically recorded for this "trail" but never used. The idea was dropped prior to transmission as it would tie stations to a fixed broadcast order - unless they were prepared to swap them around or cut them off completely.