(somber music) - So we left Cassie at the end of season three in quite a bad place.
She has been away from work because the previous case took a huge emotional toll on her.
- We find Cassie still struggling to recover from the trauma that was visited upon her.
- As a result of her brush with the evil serial killer Doctor Finch.
- The man was a psychopath and unrepentant.
Do you feel any guilt for what you've done?
- No.
- It's very hard for her to sit opposite somebody who was not asking to be forgiven.
Dad, I'm not sure what's made you change your mind.
We worked it all through, as soon as you were diagnosed.
Do you remember?
Her father's becoming increasingly ill.
- I think we've seen a fragmentation of Cassie, you know dealing with the pressures, dealing with pressure at home with her father diagnosed with dementia.
A certain degree of loneliness.
Then by series three she'd found someone.
- So pleased to see you.
- Sunny in the meantime has kind of found a partner, Sal, that he's setting up home with.
- Cassie and Sunny are very real people, and you see the emotional toll on Cassie.
- Maybe you might need to take some time off.
- Cassie suffered a nervous breakdown and she has gone on sick leave from the police.
- I don't wanna go back.
I've already given the job everything I have.
- At the beginning of series four we find that Sunny is carrying on.
You ready?
- Yeah, I'm good.
- Another case has come in and he's off pursuing that.
