Unforgotten: Season 4

Season overview

Role: Meticulous television and film archivist.
Task: Produce a JSON object for "Unforgotten" Season 4.
Keys: overview (160-220 word summary), primer (array of exactly 4 concise bullet points).
Constraints: Use ONLY provided SYNOPSIS and SOURCES. No invented facts. No markdown/asterisks/headings/bullets in the prose. Output valid JSON only.
Content:
overview: Factual summary of Season 4's narrative arc.
primer: 4 concise bullet points on previous lore/characters/stakes.
Synopsis: Season 4 starts with a dismembered body found in a scrap metal yard. The team believes it was stored in a domestic freezer for thirty years. A Millwall Football Club tattoo identifies the victim as Matthew Walsh, a man in his mid-twenties who went missing in March 1990.
Source 1: London police detectives Cassie Stuart and Sunny Khan investigate historic cold cases involving missing persons.
Source 2: British crime drama, aired on ITV from 8 October 2015. Created/written by Chris Lang, directed by Andy Wilson.
Source 3: Created by Chris Lang. Cast includes Sanjeev Bhaskar, Jordan Long, Carolina Main, Pippa Nixon. London detectives investigate crimes from the past.
Source 4: Cast includes Sanjeev Bhaskar, Carolina Main, and Jordan Long.
Source 5: Mentions Season 7 is on the way (irrelevant to Season 4 content).
Overview (Target 160-220 words):
The fourth season of the British crime drama Unforgotten begins with the discovery of a dismembered body located in a scrap metal yard. Investigators believe that the remains have been kept in a domestic freezer for approximately thirty years. Through the identification of a unique Millwall Football Club tattoo, the victim is identified as Matthew Walsh. Walsh was a young man in his mid-twenties who had gone missing in March 1990. London police detectives Cassie Stuart and Sunny Khan lead the investigation into this historic cold case. As part of the series' ongoing premise, the detectives work to unravel secrets that have been buried for years by investigating crimes from the past. The investigation focuses on the circumstances surrounding Walsh's disappearance and the subsequent discovery of his remains. This season continues the established pattern of the series, which was created by Chris Lang and features detectives investigating missing persons cases that have remained unsolved for decades. The procedural elements involve tracing the history of the victim and the timeline of his disappearance to uncover the truth behind the crime