Yes, it's a pretty obvious question, considering it's presumably the central thread of the entire series. But the solutions to the cold cases of the first and second series were so devastatingly unexpected that it's impossible not to be immediately engrossed by the primary mystery of the third. So far, all we know is that Hayley Louise Reid was was 16 when she disappeared, and that her body was discovered 18 years later in North London. But who or what killed her? How was her body found? And why did it take so long? One interesting development that a zoom in on the still of that newspaper text reveals is that Reid was apparently a twin (fourth line down, am I right?). The picture in the report shows a person who looks very similar to Bronagh Waugh's character, so does Waugh play the twin? Will there be family drama? So. Many. Questions.

Don't think we didn't notice that lingering hug, you two. You'll recall an excruciating moment in series two when Sunny tries to kiss Cassie, only to be bluntly (and hilariously) shut down. But was that really the end of any potential romance between the pair, or does this hug suggest otherwise? Friends hug, sure; colleagues too, I guess. But maybe something more is on the horizon?

"That's the closest thing to evil I've ever witnessed," says Sunny in the trailer. It's a pretty giant claim, considering in the past two series he's worked on cases involving sexual abuse, murder, false rape allegations, blackmail, and homophobia. So what could possibly be so abhorrent, in the case of Hayley Reid, as to surpass all that?

A newspaper headline featured in the trailer reads, "DEAD COPPER ESCAPES JUSTICE", with the subheading, "Tragic teen's family want answers". As we know, Unforgotten isn't a show that unthinkingly glorifies the police. In series two, DI Tessa Nixon was a suspect in the death of her then husband, David Walker; though (spoiler alert!) she was ultimately cleared, as the Guardian recaps, it was revealed she knew about his abuse of schoolgirl Ellen Price. So what is the problem in the force this time?

"I know stuff. Stuff I never mentioned before." Sounds a little sinister, Neil! According to Radio Times, Morrissey plays one of a "close-knit group of old school friends that have stood by one another through thick and thin" — and from the trailer, it sounds like they've stood by each other throughout some seriously shady stuff. What have the friends been keeping secret, and how does it relate to Hayley Reid? Also what is The Crown's Alex Jennings' character up to. He looks guilty

Just going to sound that spoiler klaxon once more before getting into this one: in the season 2 finale, as the Guardian recaps, the duo discovered that central characters Sara, Colin, and Marion co-conspired to murder each others' abusers. Deciding that there's no positive outcome of prosecuting the three — "I think their whole lives have been one long indescribably brutal punishment," Cassie says — the detectives elect instead to walk away from the case, without turning over their discovery to CPS. It's a pretty major decision, and writer Chris Lang told Radio Times, "I don’t think it’s something that they will wear lightly" — so what will the ramifications be for Cassie and Sunny? Will they grapple with guilt or regret? Will their careers come under threat? The trailer closes with Cassie looking seriously distressed about something. What exactly?

Only one week to go, Unforgotten fans! Let's hope the show provides some answers; although, if precedent is anything to go by, I imagine there'll be an awful lot more puzzles to solve by the end of the first episode.