It was created by the television producer Chris McMaster, who was aware of the popularity of adult action series such as The Avengers and Department S amongst teenagers and saw the potential of a version aimed at a younger audience.

In the opening story, three young people become caught up in the plans of ex-Nazi officer Karl von Gelb to "reverse the verdict of the last war" and inflict revenge on his former enemies.

Teams of scientists were also regularly kidnapped and coerced into building similar gadgets: devices for changing the weather, controlling minds and melting metal at great distances all featured.

Strong incidental and closing music by Laurie Johnson and a memorable theme tune, the Carnaby Street Pop Orchestra's "Teenage Carnival" composed by Keith Mansfield, helped maintain the momentum, and Von Gelb's appearances were usually accompanied by stirring passages from Wagner's Ring Cycle.

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.

[5] 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.