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*Warning - Unforgotten spoilers ahead*
Fans have been waiting patiently for the BAFTA-nominated series, starring Nicola Walker as DCI Cassie Steward and Sanjeev Bhaskar as DI Sunil Khan, to return ever since the last instalment, back in 2018.
In Monday's episode, the pair uncovered the dissected body of Millwall Football Club supporter Matthew Walsh in a scrap metal yard.
Even eerier, the team believe the body had been dumped there after originally being stored in a domestic freezer for as long as 30 years.
The freezer is discovered to have belonged to a recently deceased man, Robert Fogerty. When the team pull up Fogerty's records, they learn he was caught drink driving the same night the victim vanished.
They also discover there were four passengers in Fogerty's car that night.
Cassie and Sunny meet the retired officer who wrote up the drink driving report and he recalls that Fogerty was heartbroken when he was pulled over because he was a 'newly qualified copper' and the DUI meant his career had been tarnished.
Fogerty and the four passengers were brand new officers on their way home from a 'passing out' party.
Fans think they have figured out who the four passengers are after briefly meeting four new individuals during the episode - therapist Fiona Grayson, engaged career woman Elizabeth Baildon, businessman Dean Barton and father-to-be Ram Sidhu.
Could they all be the former police officers who were passengers during that fateful day 30 years ago?
And there's more - viewers will recall that Cassie requested an early retirement from the police following 29 years and nine months of service - just shy of the obligatory 30 years. Since the crime happened 30 years ago and it involved newly qualified police officers, was she acquainted with them at the time?
Fans on Twitter seem to think so. One woman wrote: "29yrs 9months keeps being repeated... could Cassie know them? People in the car were soon to be new officers 30 years ago then obviously changed careers after the crime??? [Were] they all involved in this murder? Where's the head?? Ahhhh so many questions."
Another fan speculated: "I think Cassie was in same intake as those in the car...and passed out with them...."
A third person said: "wait they were in the car 30 years ago and cassie has been in the force for 29 years and 9 months DOES SHE KNOW THEM??
There was lots of praise for the first episode's cliff-hanger too. "Brilliant first episode of #Unforgotten and that cliffhanger at the end! Just wow! Start the countdown until next week for episode two now!" said a fan.
Writer and creator Chris Lang previously said: "Unforgotten [series] 4 will once again explore the fallout from and the investigation into, a historic murder case, but the primary theme of this series, will be society's relationship with the police."
What a cliff-hanger!
Unforgotten continues on Monday at 9pm on ITV.
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