- Ending spoilers: So the implication is that Liz successfully framed Martyn with the key in his pocket. So the police assume he locked them in from outside with that key, which raises two serious problems. First, Liz's claim is that she picked the lock to escape. How could someone have a double-keyed lock that could be picked from either side without creating an air passthrough? Considering the Hole is explicitly a fallout shelter, being airtight is its #1 reason for existing. Second, the police's entire case hinges on Martyn being a total psychopath who intends to murder four people for no reason whatsoever, and is genius enough to cover his tracks thoroughly - but then carries around literally the only piece of evidence in the world that could tie him to the hole in his pocket when he could have disposed of it a thousand ways.
- The police assume he committed suicide out of guilt over the whole thing. The police had no other leads. Philippa doesn't discover Liz locked them in there until she goes down into the bunker with her. At which point, they've already found Martyn's body with the key. Additionally we see Geoff trying to pick the lock during a montage. Presumably he kept trying at it for a while, so the lock would show evidence of someone working at it. The detective also doesn't really like Martyn that much and seems determined to pin the case on him. It's important that Liz is never a suspect up until she confesses. And then it looks as if Philippa abducted her from her house (the smashed biscuit tin) and forced her to go back down into the hole. Additionally Martyn's parents would probably know or the school would about the keys - he had to get them from somewhere. All of the evidence points to him and none of it to Liz so why should they think otherwise?
- Also the hole is not a nuclear fallout shelter. It's a bomb shelter from the second World War. So it would have been designed with a regular lock. Sure they could have used it as a nuclear fallout shelter if they'd sealed the door up.
- There's a line earlier in the film when Martyn is being released - where DCS Howard tells Philippa that Martyn runs a website about anarchy, calling for disruption of air traffic control and the like. That just implicates him further and makes him look questionable.
- At the start of the film, we see a picture of Liz hanging on the wall looking very geeky and brainy unlike her actual persona. Is it real?
- Out of universe it's a Red Herring to make the audience think Liz is a bookish Shrinking Violet. In-universe it's possible Liz could have hallucinated the picture, as we never see another like it. Or maybe she was both personas to some extent - putting on a responsible facade for her teachers and sneaking off for drunken parties when they weren't looking. Liz is certainly smart - if sociopathically so - so maybe she gets high grades with little effort.
- It could be an older picture too - maybe from one or two years ago. Maybe Liz was the bookworm sort before and then she fell in with the popular girls.
- Why is Liz so calm about her friend violently throwing up?
- Because Liz quite frankly is a sociopath. It's the Intended Audience Reaction that her friend is literally dying beside her, and all she cares about is Squeeing about getting a kiss from Mike.
- More charitably, Frankie is said to be bulimic, and Liz is her best friend (not to mention they drink a lot). So they've possibly had many conversations with Frankie puking into a toilet that Liz is just adjusted to it by now. For what it's worth, Liz does try to unlock the door directly after this.
- Can't the psychologist just tell everyone the truth? Liz admitted the whole story to her and it would match up with the facts.
- Liz admitted the truth in private in a crowded bunker, with no evidence that they had agreed to meet there. Philippa even says that she needs to repeat everything on tape with lawyers, parents and police there to hear it. Sure it's her word against the psychologist's - but Philippa's credibility is in doubt at the moment. DCS Howard has repeatedly been chewing her out for getting too personal to Liz (going out to lunch with her etc). Liz's mother also rang the station claiming Philippa ran off with her. The audience knows that's not what happened, but Liz's mother came home to find her daughter gone and a smashed biscuit tin (possibly with footprints signalling someone running) right after she saw Philippa's car driving off. Philippa still suspects Martyn until she's down in the hole with Liz, so it looks very odd that she'll have wildly changed her tune in the space of a couple of hours. It looks like Liz was abducted from her home, taken to the scene of the crime and forced to confess.
- If you don't take the alternate ending as canon, maybe she did after the credits rolled, and spent several months or years accusing Liz of murder.
- Philippa also isn't letting anyone else question Liz (she even says to the detective "if you say one word to her, I will fucking kill you!") so by the time the police get to question her themselves, she's able to say plenty of things that will incriminate Philippa against her. The taped sessions will show Liz with severe delusions and psychological problems - so a 'confession' that she looked distressed about giving down in the hole won't hold up in court.
Headscratchers / The Hole