*Warning: spoilers ahead for Unforgotten season four*
Who killed Matthew Walsh in Unforgotten season four? The beheaded and handless human remains found in a London scrapyard belong to a football fan missing since the 1990s, and whose body had been hidden in a freezer belonging to a recently deceased man, Robert Fogerty.
Fogerty had been caught drink-driving on the very same night of Matthew's original disappearance - and as DCI Cassie Stewart (Nicola Walker) and DI Sunny Khan (Sanjeev Bhaskar) reopen the cold case, they learn that not only was Fogerty a newly qualified police officer at the time of his arrest (thus jeopardising his career when he was caught), but so too were his four car passengers.
That means that our four key suspects in the Unforgotten cast - Fiona Grayson, Elizabeth "Liz" Baildon, Dean Barton, and Ram Sidhu - were once trainee coppers in the 1990s. And we've since learnt that two of the suspects (Elizabeth and Ram) are still serving police officers, and a similar age to Cassie herself.
We've learnt more about what our four suspects were like when they were younger - and learnt the connection between the Unforgotten suspects was that none of them had friends or family attend their passing out (when they qualified as coppers).
Episode five was chock-full of revelations, from historic crimes and arguments unearthed, to new interrogation scenes. We learnt that a few weeks prior to his death, Fiona was sexually assaulted by Walsh. Ram came to her defence, but was badly beaten.
When the newly qualified coppers all saw Walsh on the night of their passing out, they chased after him. But what happened next?
Rare plant matter found with the remains means that the cops can place Walsh at a local allotment at the time of his death, while the injury to the head now suggests that he was stabbed with a fountain pen by one of our four suspects.
Let’s take a closer look at the evidence against each of our suspects for Unforgotten season four…
Fiona Grayson (played by Liz White) is a therapist who lives in the Peak District with her husband and two children. She and her husband are investing in a new business venture by setting up a practice, presumably fronted by Fiona. We also learn that Fiona had an unhappy childhood growing up, only training to join the police because her father expected her to - and on the surface she seems a sympathetic and caring character.
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She was the first suspect to crack during an interrogation scene in episode five, recounting the night Matthew Walsh went missing. A few weeks before his death, she had been sexually assaulted by Walsh, before Ram came to her defence and was badly beaten up.
She also confirmed all four of the suspects got out of Fogerty's car to chase after Walsh on the night of his disappearance. However, she insisted that she believed his death was an accident.
We've also learnt that, as a young woman, she was charged with death by dangerous driving, after killing a young boy in a speeding accident. (Her criminal conviction means that she never obtained a real licence to practice as a therapist - she's been using fake papers since the early Noughties.)
Cassie and Sunny discovered that officers at the car crash site smelt alcohol on Fiona, and took a blood test at the police station - but that blood sample mysteriously went missing. Connecting the dots, Cassie and Sunny realised that Elizabeth "Liz" Baildon was working at that same station at the time, and must have stolen the blood test.
They theorised that either Fiona and Liz were a couple, and that Liz stole the blood to protect her - or that Fiona blackmailed her old friend into stealing it.
When Sunny first tracked Fiona down for a preliminary interview, she revealed that she was unhappy in the police and became an alcoholic, meaning she 'doesn't remember' much of her time as a trainee police officer (even though a flashback suggests that, like the other three suspects, she remembers all too well).
Fiona also seems the most mysterious - and potentially the most repressed - out of all our suspects. When the discovery of Matthew Walsh's frozen remains was first detailed on the news, all of the other three suspects looked horrified and shaken when they heard the news bulletin, but Fiona remained stony-faced beside her husband.
She also met up with Liz Baildon in episode four, liaising in a park (disguised in hoodies) to discuss - in coded terms - the fateful night when Walsh was killed.
Elizabeth "Liz" Baildon (played by Susan Lynch) was only shown to be a high-flying career woman in episode one, but we've since learnt that she's a high-ranking police officer - and about to be offered the top job in the East Anglia area. We even saw her valiantly throwing herself in harm's way when she witnessed a moped theft.
She also seems to be a big-name in the police world - Cassie referred to her as "the Liz Baildon".
Outside of policing, Liz lives in Cambridge, and is engaged to be married to her partner, Janet.
However, it's beginning to look like evidence is pointing towards Liz, who, in episode five, withdrew her application for the job, and threatened to smother her elderly (and extremely sharp-tongued) mother, Eileen (played by Sheila Hancock) with a pillow.
Liz has long had a very antagonistic relationship with her mother, who previously called to give a veiled threat when she heard about her daughter's job interview.
Eileen had a conversation with her carer Eugenia (Mina Andala), during which she said she knows things about Liz that would "turn your hair grey". Eugenia used the mysterious anecdote from Eileen as a means of blackmailing Liz into upping her salary.
Could those same secrets be connected to Matthew Walsh's murder? Liz was certainly shaken when she learnt that Walsh's remains has been discovered, even dropping two china cups to the ground and smashing them.
She also lied to Cassie during their interview in episode three, and again in the interrogation in episode five, when Cassie questioned her about Matthew Walsh's disappearance and death.
She was also eerily calm when she met up (secretly) with Fiona to discuss Walsh, talking in coded terms about "the pub". The only real moment of emotion was when she roughly kissed Fiona on the cheek, before telling her that she's "still beautiful".
Were they in a relationship? Was Liz in love with her, and could that be connected to the case? And did she steal Fiona's blood test?
Living in Rochester, Dean Barton (played by Andy Nyman) is a seemingly legitimate businessman who organises charity functions (and intends to make the maximum amount of money out of them). Like fellow suspect Fiona, he left the police force.
He's happily married with a disabled adult son, but we later learn that he has a dodgy past - and certainly has the connections to theoretically help dispose of and hide a body for 30 years...
His real name was Dean Quinn, but he legally changed his name - and it's clear why. His father and brother were all career criminals, and viciously beat him when they discovered he'd applied to be a copper.
But has he truly escaped his family past?
Dean's nefarious connections were immediately established in the opening episode, when he got a call from the mysterious 'Felix' - an old colleague of sorts who is cashing in a favour, and needs help moving a "shipment". In episode two, we saw him drive to France and back, picking up a mysterious white van along the way.
So far the details of Dean's more nefarious dealings have still been kept in the dark, but given that we now know that Dean was once a qualified police officer before leaving the force, there are even more questions yet to be answered.
In episode four we learnt that Ram was somehow connected to the "shipment" - the pair met up in a wooded area to discuss what they'd told the police about Matthew Walsh, and at the end of their conversation Dean thanked Ram for helping him (perhaps by telling border control to turn a blind eye?).
Dean also proved a "lovely" interviewee when Cassie spoke to him during episode three - but as he later tells her, you never really know what someone is capable of...
It also seems that Dean is by far the most private and secretive of the four suspects.
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In Southall, expectant father Ram (played by Phaldut Sharma) is - on the surface - charming, impeccably dressed, and known and liked locally by everyone. However, at his core there seems to be a sadness and anger at the world.
He, like fellow suspect Elizabeth "Liz" Baildon, remained in the police, and he's now the same rank as Cassie. However, he's been accused of sexual harassment in the workplace, an accusation he initially brushes off, before claiming that the complaint was racially motivated. He's determined that his young female accuser should back down - but when she doesn't, we see him secretly searching her name in the police database in order to blackmail her with any past criminal record.
He also reacted aggressively to Sunny's line of questioning in episode three, even using the slur "coconut" (suggesting that he believes Sunny is 'white on the inside,' and somehow working against him).
In episode four, we learnt that three weeks prior to Matthew Walsh's disappearance, he (Walsh) had beaten Ram in a brutal, racially motivated attack, but got away with it.
That attack took place in a pub; and weeks later, it was Ram who first chased after Walsh, before the other suspects got out of the car and followed him.
Ram shared a moment with his brother in episode one, when he expressed bitterness about his past and reveals how he once felt constantly underestimated. It's clear now that Ram has a chip on his shoulder that can be traced back to that pub attack.
However, that also gives him motive for revenge against Walsh.
After all, we've since learnt that a man matching his description was seen chasing Matthew Walsh on the night of his disappearance, accompanied by an "unusually tall" third man (Fogerty) - and that they both knew Matthew Walsh by name.
Unforgotten season four continues on Monday nights at 9pm on ITV. Want more show content? Read the latest Unforgotten episode 5 ending explainer. Looking for something to watch? Visit our TV Guide or find all the latest news on our Drama hub.