Still wondering about the questions asked above, I searched online just now and have found mention, on Wikipedia, that Duncan Hunter is Cassie's biological father. However I remain fully unclear whether he fathered Cassie during her mother's marriage to Jimmy, or if Cassie was born prior to that marriage, or where/when/how exactly Duncan fits in the picture.

Can anyone explain any of that, plus what became of Jimmy Perez's wife (Cassie's mother)? Was Duncan ever married to her, or had he and Cassie's mother strictly been an item at some point?

Some more will be revealed in series 4.

The series is based on novels by the author Ann Cleeve.

As often is the case, the books and this series differ on many points. One example is the main character, Jimmy Perez. In the books he is of Spanish ancestry with dark hair and olive skin (a black Shetlander).
In the series he, well, looks different.

I know from the books that Fran was married to Duncan Hunter and they had a daughter, Cassie. They seperated (and later divorced) and after a while Fran and Jimmy got together. In the books she was killed before they got married. Jimmy took care of the young Cassie, while Duncan was mostly out of the picture.

In the series, it seems that they did get married, so Jimmy is Cassie's stepfather. It hasn't been explained here - as far as I know - how Fran died. Only that Jimmy returned to Shetland from Glasgow and is raising Cassie together with Duncan, who is now married to Mary.

The relationship between Jimmy and 'chancer' Duncan largely revolves around the upbringing of Cassie. There are tensions and they'll have to find a middle ground for their daughter's sake, who still misses her mother.

Side note:

• Another detective series adapted from Ann Cleeve's books is the popular crime series "Vera (2011-)".

• Off-topic: I noticed on the "Grantchester (2014-)" board that you had some questions. In my second post here I had a little rant about Sidney and his 'affairs'.

I agree in large part with @merryapril post about watching series 1-3 to gain more insight into his relationship with Amanda.

It seems like the writers sent him off in a manner contrary to his own characterization and beliefs in the previous series, unless they wanted to accentuate how unbalanced he had really been all the time and this has now culminated in his final actions.

Vera is a series I've caught some bits and pieces of (and most of one particular episode), that I've liked what I've seen of. One of my t.v. stations runs seasons of it now and then.

Thanks so much for the excellent info about the Duncan-Cassie-Jimmy dynamic! That definitely tremendously helps make better sense of all that for me. Although I've not missed any episodes, or even any parts thereof, through the point I'm up to now, many explanatory details of early on clearly went over my head.

Now I'll no longer feel mystified whenever Duncan appears or is referred to in an episode!