- On this season of "Unforgotten," there are body parts found flung into a marsh.

- We start off with a bit of a spine, so not much to go on.

- My guess is this body was put in there already dismembered.

(tense music) - Realistically, how far could someone throw bits of a body?

- Let's say they did your basic dismemberment.

- Head, arms, legs, torso.

- Knees and toes, knees and toes.

- This season, Sonny and Jess' relationship, it's on an upward trajectory.

- They're beginning to trust each other.

As a working relationship, they're a lot more on sync.

- They're both more settled, but there's a lot of unresolved issues in their lives.

We find him very single, quite lonely, and we find her still wrestling with the fallout in her personal life.

- To what do I owe this?

- Nothing.

- [Sanjeev] And then the pathologist finds details that point to the cause of death.

- We have a match.

- The victim we find out is a man called Gerard Cooper.

- And we grow to find that maybe Cooper wasn't a very pleasant man, which makes it even more intriguing because then everyone has a reason to have committed this murder.

- We have four lead suspects in this case.

- [Chris] We've got a ex-tabloid journalist, a university lecturer.

- [Sanjeev] A young man who's autistic and an Afghan refugee.

- You think how these people's lives are going to intermingle and then eventually they do.

- Probably one of the wildest series.

There's so many twists and turns.

- It's entertaining and it gets you thinking.

- And it's not heroes and villains.

There are shades of gray all over the place.

- You do not want the weight of this on your shoulders.

Trust me, you do not.

- If you say anything to anyone, you'll regret it.

- After 11 years of making a show, this is the best series yet.

- If you could have done that, you could do anything.

(dramatic music)