(suspenseful music) - The murder this season actually span the longest time period of any of the Unforgotten series so far.

The events that precipitated it were over 50 years, half a century ago.

So in many ways, it's the most unforgotten story we've done.

And the four antagonists, the suspects, and indeed the victim are all connected in a very unique way.

- Weird cast this year.

Because normally they're all a sort of similar age.

It was wonderful, this year, when I first read it, I thought, wow, great.

Yeah, look, they're all different ages.

And it's an intergenerational story.

That was great.

It was really good fun to do.

- My name's Ian McElhinney, and I play Lord Anthony Hume.

Lord Hume is now a member of the House of Lords.

So as a Tory minister who's had a bit of a conversion in his life, at one time, he would've been quite hard line and probably would've supported austerity back in the day.

But in more recent years, he seems to have gone through some sort of transformation in his political thinking.

- My name is Max Rinehart and I'm playing Karol Wojski.

Karol, he's moved to Paris and he's got a wonderful girlfriend there, Elise.

He's just trying to set up a life for himself.

He was living in London for 24 years.

He's run away from something and that may or may not come back to haunt him.

- My name's Rhys Yates, and I play Jay in Unforgotten.

So Jay is a young guy that had quite a dysfunctional upbringing.

When we first meet Jay, he is staying in a council flat.

He's staying with his girlfriend.

And basically, they're both suffering with drug addiction problems and they're kind of stuck in a vicious cycle, which he ultimately wants to get out of.

So Jay gets drawn into the investigation because, I can't really tell you.

We'll see what happens when you tune in.

- I'm Martina Laird, and I play Ebele Falade.

When we meet Bele, she has spent the last few years of her life really trying to pull things together, really trying to kind of, you know, get past her original circumstance.

She's trying to be ambitious, but I think in her context, it's sort of going, no, I too can thrive.

For her, that idea of esteem is a battle.

Bele becomes a person of interest to the investigative team because they managed to search out a connection between her, the body, the location, and of course, that's what the show does so beautifully, is to just take these connections in all kinds of directions that you'd never expect.