As with all good television mysteries, the acting by and the interaction between the principal stars is crucial to making the shows work.
Detective Senior Sergeant Mike Shepherd, Detective Kristen Sims, and Detective Constable Sam Breen are all unique enough characters that I never tire of watching them perform their duties and interact with each other.
The actors themselves, Neill Rea, Fern Sutherland, and Nic Sampson, as well as the writers of course, are a big reason for why these characters work so well together and apart.
Another reason I find this ongoing mystery series working so well is that, like Jessica Fletcher's Cabot Cove, Brokenwood is a small enough to have recurring secondary characters who pop up in various episodes.
But unlike "Murder She Wrote" or any other mystery show I can think of, the writers very cleverly make most of these secondary characters undesirable, one way or another.
Sometimes the secondary characters are just witnesses, sometimes they are suspects but end up being not guilty. But because they are so undesirable, when they appear in the future episode we and the detectives recognize them, and immediately suspects they may be guilty, if not of murder, of something related to the murder.
There are also subtle connections being drawn between events and relationships from past episodes, and murders in the current episodes.
These are very clever way to keep viewers invested in the show, especially since in one past episode, an undesirable secondary character from a past episode, does turn out to be the murderer in the new episode.
This episode was nicely complicated in several of the ways mentioned, as well as other ways--making this an enjoyable episode to watch.