In 1952, Boris Karloff filmed a portmanteau film consisting of three Colonel March stories to be shown to television executives as a television series pilot. The film was immediately released in the UK as the theatrical movie, Colonel March Investigates (1953). Karloff returned to England in 1953 to film more episodes once the series was approved.
At the time the pilot episode was filmed in 1952, Karloff was 65 years old and had been away from England for several decades. He was looking for a reason to spend more time in the UK at this point and was hoping he could go into semi retirement so jumped at the chance to work near to London and take the opportunity to look around and visit old friends. However, despite his advancing age and health problems, he was still so highly regarded as an actor that he was still being offered decently paid work on both sides of the Atlantic and continued to work right up until his death in 1969.
Many viewers were surprised to discover from this series that Boris Karloff's own speaking voice was actually a clipped southern English accent (as Karloff had been working in the U.S. for several decades at this point). Many of the actors who appeared in this show were also surprised that Karloff also had dark skin when they met him, a result of his Anglo-Indian heritage. Many of the guest stars commented in later years how extraordinarily polite, professional and well mannered he was, as well as having a mischievous sense of humor and rarely fluffing his lines.
The series was made from 1953-54 in anticipation of a commercial TV channel being allowed in the UK, although the licenses for commercial television (the ITV network) were not actually granted until 1954. By the time the first episode was shown in the UK (September 24th 1955, two days after ITV had started broadcasting) this series had finished filming a year beforehand. It actually had its TV debut in the US starting in December 1954 but it was made partly with the UK market in mind as the producers knew the fledgling ITV broadcasters would be desperate for quality content that was readily available for broadcast.
Producer Hannah Weinstein was an ex-pat American who moved to the UK so she could continue working in film and television after her US career was ruined by Senator Joseph McCarthy and HUAC blacklisted many people who sympathized with left wing politics. Several fellow Americans followed her to the UK (such as Phil Brown and Sam Wanamaker ) were involved in this and other projects she worked on in the UK.