The other reviews will tell you that this is one of the best TV series ever, much under-appreciated, and worth your time. They are mainly right, with a few exceptions. First the genesis of the series is odd. Callan was originally a successful novel (younger readers, this would be in the days of paper and ink, not portable devices) and a solid action-thriller. The original Callan, as written, did indeed have a conscience but always preferred a quick and reliable solution (guns and bullets) to soul-searching. When the author, Mitchell, was approached to turn the story into a TV series, he did something very unusual. Where other authors will usually blame someone else for interfering with their work, Mitchell re-imagined Callan entirely on his own initiative, turning a man of action into man of conflict. With hindsight it was a brilliant decision. While we may never know what the British public would have thought of Callan as originally conceived (remember that TV violence was very stylized at the time, look at the Avengers, or Batman in the US) they simply fell in love with the re-imagined Callan as a soul-searching and reluctant spy. The show was #1 for several years and ultimately a much pudgier Woodward shuffled off to America to attempt to re-create the Callan persona for US tastes. With mixed success.
