‘Almost breaches Rule 4,’ she said, absent-mindedly. ‘Requires scientific explanation. Not sure if a snow tunnel counts as a secret passage though.’

I’d been writing my how-to guides for a long time, so Erin knew Ronald Knox’s rules just as well as I did. I wondered if she was spouting them off now in an attempt to make us feel like a team. It seemed strangely possessive from a woman who’d lied so boldly in order to avoid having children with me. And who’d nicked my loft.

‘That’s exactly the problem,’ I said. ‘Those murders are headline grabbing. They’re perfect for the front page – they’ll be a streaming documentary in a few months. They’re statements. Which means they’re easy to copy.’

‘You’re saying maybe someone just wants us to think the Black Tongue is up here?’

‘What’s more believable, that an infamous serial killer followed us up here, or that someone’s trying to make it look like they did?’

‘Sofia’s been trying pretty hard to convince people with all her explanations,’ Erin observed. ‘It’s almost like she’s trying to scare us.’

‘She is a doctor. She treated one of the victims. It’s not as if she’s said anything that isn’t in the news.’

‘Sounds like you’re defending her.’

‘Gotta trust someone.’ I decided that was a bit cruel and changed the topic. ‘Tell me something: how did Michael convince you to go graverobbing with him?’

That caught her off guard. ‘Well, I didn’t know it was graverobbing at first. He kind of sprung that on me.’

‘How did you get involved in the first place?’ The double meaning of the word involved ballooned and filled the room.

‘Michael and Lucy were having their financial problems. You and I – we’d been struggling since . . . Well, they say a problem shared is a problem halved. It was comfort, Ern. Just comfort.’ I hadn’t meant it that way, but I couldn’t bring myself to stop her talking. ‘It’s like snow on this mountain, is the only way I can put it. Lots of little flakes, and then you’re knee-deep in it. Or ash on a lung, I guess. Is that too dark? Things seem to only move little by little, but then you look back and they’ve moved a lot. It was only when you and I had started sleeping in different rooms, but Lucy didn’t know.’

The revelation that they’d been together longer than I’d thought, before Michael had shown up in my driveway, should have crushed me. I’d had so many parts of me crushed that day, however, that it kind of bounced off.