A prequel, essentially an expanded remake of the original Callan pilot 'A Magnum for Schneider'. In this film Callan meets Dr Snell for the first time whilst in the Callan TV series he is already familiar with him. 'Wet Job' is the sequel to the TV series showing what happened to the Callan after he left the Section.
Additionally the film could be seen as what is now termed a 're-boot', since it is a remake of an earlier television episode, different actors play TV regulars and so on.