DCI Cassie Stuart (Nicola Walker) and DI Sunny Khan (Sanjeev Bhaskar) are getting closer to the truth about what happened to Matthew Walsh, whose decapitated body was found in a freezer.

But even in the penultimate episode, they were still wrestling with some big questions.

Is the killer one of Rob Fogerty, Ram Sidhu, Liz Baildon, Fiona Grayson, or Dean Barton? Or is it all of them? Could it have been an accident?

For a refresher on the last four episodes, see our previous recaps:

What happened in episode five?

Cassie, who resents the police force for rejecting her retirement request and whose father’s dementia is worsening, was still showing determination to solve the case.

There were many new developments in the hunt for Walsh’s killer. The first big news is that Dean Barton (Andy Nyman) isn’t who he said he is. His real name is Dean Quinn. We found out that he changed his surname by deed poll about three months before he applied to the police academy in Hendon. The team also discovered Dean came from a criminal family.

Ram (Phaldut Sharma) tried to calm down his wife Anna (Clare Calbraith) after she realised that he was interviewed by the police about Walsh’s murder, causing him to change his mind about keeping their baby. He reassured her that he didn’t do anything but is worried the police will frame him to get rid of him.

A huge breakthrough came when DS Murray Boulting (Jordan Long) tracked down the pub landlady Monty, who told him about the altercation Walsh had had with Ram a few weeks before his death. She revealed that Fiona (Liz White) was the one who was sexually assaulted by Walsh. She was with “Busy Lizzy” (Susan Lynch) and had a drink problem. Ram intervened and “bit off more than he could chew”. Walsh made “a real mess of his face”.

Fiona’s partner Geoff (Daniel Flynn) confronted her and discovered she had been working with no licence as a therapist for 16 years. It put their commercial loan at risk. She admitted the regulatory body wouldn’t have issued her with one due to her dangerous driving conviction.

Another crucial bit of evidence linked Ram and Dean with phone mast data. It put them in a wood near Guildford three days after the announcement about Walsh’s body being discovered, despite them both claiming not to remember each other.

With no wounds, kick marks, or punches on the victim, Cassie concluded that only one of the suspects killed Walsh and the others came across the scene.

Cassie’s game plan was to “divide and rule”, dangling cooperation deals with the suspects during further interviews. Would they all blame Fogerty, “the dead guy with a body in his house”?

An old man came forward with information. He remembered seeing a group of people loading something heavy into a car boot by the allotment. He had asked them at the time: “What you got there? A dead body?”

Liz gave her mother’s carer Eugenia (Mina Andala) a bigger pay rise than expected to keep her quiet about incriminating evidence that Eileen (Sheila Hancock) had told her. But to Liz’s surprise, Eugenia was apologetic about threatening to report her to the police and thought she deserved better than to have Eileen speaking badly of her.

Liz took her mother a cup of tea and ended up threatening to kill her. “You will speak to me with respect from now on or I swear, I swear, I will come in here while you are sleeping and stick a pillow over your smug self-satisfied face until you are quiet,” she warned. She wanted to know how long her mother had known the incriminating information, which she’d told her father on his deathbed. We still didn’t find out what it was – did she admit to her father that she had killed Walsh?

Cassie and Sunny paid a visit to Dean’s mother, who said she hadn’t seen him for 30 years. Dean betrayed his family by becoming a copper, according to her. She said she is now desperate to see her son as she has a terminal illness.

The team discovered that Ram is capable of violence and has twice been accused of using excessive force.

They also couldn’t prove that Liz stole Fiona’s the blood test the night she was drink driving, but we discovered that Liz was on custody duty when the vials disappeared and she had volunteered to do overtime that evening. Cassie called the pair into questioning.

As Liz saw Boulting and DC Jake Collier (Lewis Reeves) arrive at her police station to bring her in for questioning, she told her superior she wanted to withdraw her candidacy for the top job as Chief Constable of East Anglia Police.

During the interview, Liz was caught out when she claimed she last saw Fiona when they left Hendon. Cassie confronted her with evidence that she lived with Fiona in Thames Ditton when she was stationed at Kingston police station.

Liz confirmed she was in an on-off relationship with Fiona but said she hadn’t seen her since they shared a flat nearly 30 years ago, despite having just met up with her in a park for emergency talks.

Cassie went in for the kill and asked her why she stole the blood samples taken from Fiona when she was arrested for dangerous driving. But Liz looked blank. Liz said they had been separated for a year when Fiona was arrested. Liz didn’t want to cooperate with any plea deal.

Dr Leanne Balcombe (Georgia Mackenzie) discovered that the object in the victim’s head was a posh fountain pen. The new evidence pointed to Walsh’s death being murder and not an accident. It was suggested that the pen might have a serial number that will give them a link to the person who purchased it.

Fiona held a dramatic house meeting with her children and partner to confess her endless lies. But as the police pounded on the front door, she didn’t get to tell them the worst lie – presumably her involvement in the Walsh case.

“You’ve come about the body haven’t you?” she said as DC Fran Lingley (Carolina Main) took her away to the station. Her family looked shell- shocked – quite understandably.

During Fiona’s interview with Cassie and Sunny, she told them that Walsh’s death was an accident. He wasn’t stabbed with a pen. She said Ram chased him to search him for drugs for “a bit of fun” and to “scare him”. Rob then chased Walsh and Ram, Dean followed, and finally Liz and Fiona caught up, to find out what was happening. Ram gave Walsh CPR and said not to call an ambulance as Walsh was already dead. He suggested they move the body. He was worried that even though it was an accident, it would get traced back to him because of the fight in the Ifield pub and fibres of clothing left on the victim’s body. Fiona said she would never have covered it up had it not been an accident. Cassie and Sunny thought Fiona was telling the truth unless “she is the best liar in the whole world”.

At the end of the episode, Cassie was involved in a road traffic accident on her way to the mortuary to see Balcombe. She had just picked up her mobile phone to check if her father had called while stopped at the lights.

Is it conceivable that one of the suspects drove into her on purpose? We saw all of them in cars before the crash.

Then out of the blue, another car ploughed straight into Cassie. Everything went black.

What have we learned about the main suspects this week?

It’s looking like Liz could well be the killer. Did she seek revenge on Walsh for sexually assaulting the woman she loved, Fiona? When she threatened to suffocate her mother with a pillow, was that a sign Liz is capable of murder? Liz lied outrageously to Cassie and Sunny in both interviews. We know she was on duty in Kingston the night Liz’s blood sample was lost and that Fiona would have ended up in jail if it had been tested. Liz doesn’t want to cooperate with police in a plea deal. No comment is her catchphrase.

Rob hero-worshipped Ram and that’s why he agreed to take the body, according to Fiona. He was only over the limit for drink-driving after Ram gave him a few swigs of whisky because he was so shaken up at Walsh’s death. We now know Rob was a very sad man who worked in a variety of dead-end jobs for 10 years after leaving the police force. Then his drinking problem took over and he couldn’t hold down work. He lived on benefits and then inherited money from his father. A teacher at his old secondary school described him as a “very sweet boy who struggled academically” but was also “easily led”.

Ram is the most “obvious” candidate to be the killer, according to Cassie. Fiona testified in her second interview that it was Ram who led the chase after Walsh and she found him giving CPR to Walsh once he was dead. Ram told her it was “an accident”, but how do they know? He told them not to call an ambulance and suggested they take the body. Was this because he didn’t want to get found out? We also know Ram is capable of violence. He’s twice been accused of using excessive force. He was cleared on both occasions but witnesses described the violence as “coming out of nowhere” and say he went from “0 to 60 in under a second”.

Dean looks like butter wouldn’t melt, but is more shady than that. He is not Dean Barton but Dean Quinn. When he became a copper, it didn’t go down well with his criminal family who beat him up. But nothing links him to the pub or Walsh.  When his estranged, dying mother turned up to apologise and tell him she loves him, Dean broke down in tears and begged her not to make him go back to his family, whose values he hates. He’s been trying to distance himself from his family for 30 years. But we know Dean is in cahoots with Ram over Walsh’s death. When will the team discover his dodgy criminal past?

Is Fiona telling the truth? Was she covering up an accident, not a murder? Cassie and Sunny think she is innocent. But she could get a PhD in lying. She bombarded her shell-shocked family with so many of her secrets, including her dangerous driving conviction, her past career as a police officer and her lies about being put up for foster care. She tried to kill herself twice and was an alcoholic. She always expected to be found out about practicing as a therapist fraudulently, but it never happened. She pretended her father died when she was a baby to hide the fact he was a copper. In the interview, she said she was the only one who didn’t help load Walsh’s body into the car boot as she was too upset. The horrific decision to go along with it so Ram wasn’t implicated has haunted her, she said.

What lines of enquiry are yet to be explored?

What were two of the suspects (a man and woman) arguing about when the witness saw a group of people loading something heavy – presumably Walsh’s body – into a car boot the night Walsh died?

Why did Dean, whose family are career criminals, join the police? What’s the story there?

What happened to Dean’s fourth brother who died? Cassie wants to know.

What do the suspects’ bank accounts tell Cassie and Sunny about them?

What does Liz’s mother know about her daughter?

Why is Liz insisting she hasn’t seen Fiona in nearly 30 years?

Does the posh fountain pen have a serial number that leads them to the person who bought it?

Why won’t Ram’s father look at him? Why is he so ashamed of his son?

Is one of the suspects trying to bump off Cassie?

Unforgotten airs on ITV at 9pm on Mondays.