Ronald Leigh-Hunt, as the British secret service agent Colonel Buchan, was the most regular cast member, appearing in series 1–4 and series 6. In 1971, he had a major part in the feature film
Le Mans and when production of that overran by two months, he was unavailable for series 5; the absence of his character from the series was explained as him being on an overseas mission. The teenagers, almost invariably referred to as "the kids", varied over the years. The original line-up consisted of
Tom Owen,
Mary Maude and Gregory Phillips.
Chris Chittell featured in series 2 and 3 and
Adrian Wright played Mike Hobbs in series 4–7.
Carole Mowlam played Fiona in series 3-4. Former
Doctor Who companion
Wendy Padbury joined the cast in series 5 as art student Sue Craig. Her youthful looks enabled her to portray a character several years her junior, although by the final series, with the actress in her mid-twenties and then married to actor
Melvyn Hayes, careful camera angles were required to conceal an advancing pregnancy. Padbury was the only regular actor to appear in the eighth series, which was a single 13-part serial. She was teamed with co-star Martin Neil, who played Sue's boyfriend Dave.
Caroline Ellis replaced Padbury in 1972 for series 7 only, playing Jill Rowles. Initially
Geoffrey Toone played the resident villain Karl von Gelb, a former high-ranking
SS officer. To avoid prejudicing potential sales of the programme to
West Germany, references to the character's Nazi past were dropped in series 3. From series 4, the character was replaced by a succession of similarly crazed criminal masterminds, such as
Jerome Willis as Professor Nero and Commander Caine played by
Kevin Stoney. The criminals were aided by various henchmen, perhaps most memorably by Ryan (
Richard Shaw) and the beret-wearing Burke (
Michael Ripper) in series 5 and 6. ==Episodes==