Ordeal by Innocence episode 2: everyone's a suspect as the BBC Agatha Christie mystery deepens

Episode two of Ordeal by Innocence certainly ramps things up, teasing us with new revelations and flashbacks to keep viewers guessing.

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So many of the characters are so deliciously vile that it's easy to imagine any of them wielding the murder weapon, but the truth still eludes us. What really happened on the night of Rachel's death?

Before Sunday's final episode, here's what we learned in episode two.

Who really is Dr Arthur Calgary?

As the Argylls noted from the beginning, something about Dr Arthur Calgary's story didn't add up: how could he be a physicist who'd been in the Arctic for 18 months, when he couldn't even remember the type of car he'd been driving, where he was going or what he was wearing? However, the truth isn't what anyone was expecting.

After jumping off a train, calling the police and surviving an assassination attempt, Dr Calgary (Luke Treadaway) makes his confession to Hester: "I am telling the truth, but I have been lying. On the 24th December 1954 I was admitted to a psychiatric institution against my will. I got out. I stole a coat and I stole a car and absconded. I met your brother on the road, I drove him to the town and I was caught. Of course I was caught. I don't know why I thought I could get away, I wasn't myself, I was quite distressed.

"And that's where I've been for a year and a half. Not in a research station, not working. I've been in an asylum. I was released yesterday."

Dr Calgary had suffered a mental meltdown over the role he played in the development of a nuclear weapon: "I wrote a formula, part of a formula, a tiny part of a formula, for detonation of Fat Man and Little Boy. for The Bomb. I'm damned."

But now he wants to do one good thing, and clear Jack's name.

Is Kirsten the killer?

As Philip (Matthew Goode) says with a sneer, it's always the faithful retainer who knows more than they're letting on. In this case, Kirsten (Morven Christie) may be our murderer – or, indeed, double murderer.

At the end of episode two Philip is discovered dead, collapsed in the shower with a morphine syringe still embedded in his arm. No one will mourn him, but viewers know that Philip was actually murdered as he tried to escape the house in his wheelchair, turning to face his killer with the final words: "Of course it's you."

Kirsten has been creeping around the house, increasingly furious at Philip's attempts to uncover the truth and listening in on his phone calls. And it's Kirsten who had been so alarmed by Dr Calgary's arrival that she kept running to the nuclear bunker to hyperventilate. Could it be her? At this point in the night, there are only three people in the house: Leo, Kirsten, and Calgary (locked in to his own room).

It may also be Kirsten who killed Rachel: after all, she was the one to "discover" the body as the blood was only just beginning to pool on the floor, and now we've seen flashbacks of Kirsten confronting her boss while holding a hefty stoneware bed warmer, with the tearful words: "You promised."

The only seed of doubt is that surely this is too early in the drama for us to know the killer's true identity. Is housekeeper Kirsten Lindstrom a red herring?

Was Jack framed by the police?

A big piece of the jigsaw puzzle: it looks like Jack Argyll (Anthony Boyle) was probably framed by police boss Bellamy Gould (Brian McCardie). But Bellamy's hatred of Jack was so evident that you have to question why Leo (Bill Nighy) never considered this possibility, especially as his old pal Bellamy had previously declared: "I'll have his head kicked in half, every bone in his body broken..."

Bellamy may actually be the worst person in this whole sordid story. The grudge began when Jack revealed he was sleeping with Mrs Gould (Frances Grey), before confronting Bellamy with a kiss and essentially accusing him of paedophilia and sexual abuse: "Bit too old for you aren't I? Pervert... he's very keen on orphans and neglected children, he's always happy to help them."

So when Rachel turned up dead, Bellamy immediately took Jack's fingerprints and had him arrested and charged.

In episode two, after Dr Calgary turned up to prove Jack's innocence and contacted the police station, Bellamy became a mess. He jumped in his car and tried to run Calgary over, accidentally killing himself in the process. Oops.

"I was being shut up, got rid of," Dr Calgary explains to Leo. "You can't trust the police. They're covering for someone."

Mary hates her adoptive siblings with a passion

Considering vile Philip's emotional abuse and cruelty, you'd be tempted to feel sorry for Mary (Eleanor Tomlinson) – if she wasn't such a piece of work herself. Mary was the first of the Argyll kids to be adopted and has been quietly furious ever since the other four arrived at Sunny Point.

"I am different, I am better, I was first, I was chosen!" she screamed at the late-night Argyll sibling showdown, deep in the forest where no one can hear them. "You ruined it! You all ruined it!"

Mary reckons Tina is a dirty and devious ex-bedwetter, Mickey is a freakish self-harmer, and Hester is damaged goods. Frankly she's so full of anger and bile that she could easily have had a hand in this whole affair.

Tina and Mickey are being super shifty

Something strange is going on between Tina (Crystal Clarke) and Mickey (Christian Cooke). Though they used to be "thick as thieves", Tina banned Mickey from the family home after the murder – and was furious when he turned up for the wedding 18 months later.

But despite Tina's anger, Calgary's revelations have forced them into a corner. If Tina's car was outside the house on the night of the murder, that means she was lying about being at her bedsit in the village "listening to records and wrapping presents." And if Mickey was with her that night, he certainly wasn't at work as he previously claimed.

The siblings' explanation is that they were driving up to the house to ask Rachel to change Tina's job at the library, but changed their minds and turned the car around. That is certainly a lie, but what's the truth? We've seen footage of Mickey getting out of the car and yelling at his mother, but the full story still eludes us.

Could a love affair between the (thankfully non-biological) siblings explain their strange vibe?

Something happened to Hester that night

"I think such terrible things sometimes, Tina," Hester (Ella Purnell) says as she sits in the graveyard. "What I'd like to do to people. Smash their faces until they're just blood and teeth. Anyone can snap, can't they? I could have killed her that night for what she did. I wanted to kill her. I wanted her dead."

But despite these totally normal and completely fine comments, it doesn't seem Hester is our killer. Instead, it's becoming clear that Rachel did something to Hester just before she died, and all the other siblings know about it.

In flashback we see her bundled up in the backseat of her mother's car, feverish and sweating, telling Rachel "I want Daddy". We also see her with a bloody nightdress as she hears her mother being murdered. Her siblings allude to the fact that Hester was so ill they "thought she was going to die too," while cruel sister Mary taunts her for apparently being "shop-spoiled" and throwing herself at men.

So what's going on? It's a wild guess, but perhaps a forced abortion gone wrong?

Gwenda is hiding something

Despite her good looks, Leo's bride-to-be Gwenda (Alice Eve) is so cartoonishly grotesque that surely she has to be mixed up in the murder somehow. The family's former secretary definitely has a motive: with Rachel out of the way, she could turn her affair with Leo into a very lucrative marriage.

As chaos unfolds in the Argyll household, Gwenda also seems extremely keen to march Leo to the church and tie the knot as tight as she can. Is she afraid he'll bolt if he uncovers her secrets?

She declares she "wasn't even in the house that evening" and so couldn't have killed Rachel, but we've seen flashback footage of her covered in blood and standing frozen as Rachel wailed in the distance. (To be fair, it could have been her own blood – did she and Rachel come to blows about the affair?)