The fourth season of "Unforgotten" focuses on the murder case of a man named Matthew Walsh whose headless body is found in a freezer in a trash heap. Cassie is forced to come back to work from her sick leave, after having a mental breakdown in Season 3 due to the extreme trauma of that season's dark case. Along with that, she has to deal with her shifting relationship with her father after his dementia diagnosis. Sunny seems to be in a good place at work and in his relationship with Sal, but he is justifiably worried about Cassie.
Throughout Season 4, the truth of what happened to Walsh comes to light, revealing that Walsh was not so innocent himself, and he committed quite a few serious crimes shortly before his death. But it was his choice to kill a man named Stephen, brother to suspect Dean Barton (Andy Nyman), that led Dean to get revenge and murder Walsh. Shortly before the truth is revealed and the guilty parties arrested, Cassie is T-boned by a speeding car and suffers a serious head injury. While Sunny finishes their investigation, the doctors try to save her, but she dies in surgery.
Going forward, Sunny and the rest of the team will be grieving the loss of Cassie, while also adjusting to a new partner (whoever that may be). It will definitely be an emotional time, but the detectives must keep working, and it's anyone's guess what sort of case they will face in Season 5. According to a recent interview Lang gave to PBS' Masterpiece 50, "It's the most political of the story so far, 'political' with a small 'p.' It really addresses where the UK is as a country politically, and it obviously introduces a new partner in crime for him, which is just lovely and refreshing. It's a woman, and she's pretty different from Cassie, that's kind of all I can say. She's at a very different stage in her life. She's younger. She's his boss again."
Sounds like we can't wait to hear more.
