I agree, it is a brilliant series, I have been watching it since it started and as Pearl says there IS a complicated backstory that matures with every episode.... you definitely need to see series 3 to understand some of the nuances especially related to "Tosh" in Series 4, but that said, you CAN just treat every series as a 1-off if you want, but you miss a lot of the cast's interactions
It isn't a whizz-bang shoot-em-up type show, it is actually very slow but NOT boring, there is always something happening and you do need to pay attention as plot twists and surprises catch you out. Not your typical cop show.
I certainly didn't see the offender(s) coming in this latest series...well I sort of saw one but got the victim wrong.
I have to say I was a bit disappointed with the ultimate outcome of the prime suspect, it seemed a bit of a cop-out to me, which is something the show is not known for, but it did tie up the whole complicated situation.............................................Watch it to understand!
WARNING........as the name suggests, the show is set in the Shetland Islands, Scotland. As such there are a lot of Scottish accents heard and you may need to acclimatize and "adjust" your hearing accordingly to understand everything.
Having said that however, it shouldn't be too difficult, in general it is not as thick as deep Glaswegian, for example. I think the producers have softened it a bit so international viewers CAN understand what is being said.