This was the first series the BBC made as an international co-production. UK prints credit Taurus Films as co-producers, whilst the German version refer to ZDF (Zweites Deutsches Fernsehen). This was very much due to creator Francis Durbridge's popularity in Germany (although the author himself did not write for the show) and allowed the series to venture overseas and widen its casting base. The videotape masters were apparently copied onto colour film to air in Germany.