Final appearance of original character Sgt. Johnnie Johnston.
This is the first episode in which a uniformed inspector is seen at Denton Station, when the Bainbridges' house is being searched and Frost tells him, "I need that knife." Until now, every uniformed manpower decision has either been made by Mullett or one of the station sergeants, with no uniformed officer ever seen or mentioned of the ranks in between. As Denton C.I.D. has a detective inspector and detective chief inspector, it would at the very least have two uniformed equivalents of inspector (as one would always need to be on duty for serious incidents not requiring C.I.D.) and one of chief inspector. This is a carry-over from a recurring theme in the novels on which the series is based; there are no senior uniformed officers ever mentioned above the station sergeants apart from Mullett - Frost even seems to be responsible for the entire station on many occasions when Mullett is out for matters that have nothing to do with C.I.D. The sole exception is an Inspector Harrison whose retirement party is held in the second novel "A Touch of Frost" (which was changed to Sergeant Brate in the series).
In his third appearance in the show, Clive Barnard has now been promoted to detective sergeant.
The mermaid quote from the story given to Helen Tudor was from Oscar Wilde's The Fisherman and His Soul written in 1891.