Sitting back and folding his arms, DCI McManus huffed out an I-should-have-seen-this-coming sigh. “So, you do think there’s a conspiracy.”

And again, Liza felt that strange rush of fear-excitement. That strange sense that she was doing something at once remarkable, and deeply banal. “Not exactly. But I do think improbable things happen sometimes. It’s like—back in 1998 there was a woman called Sally Clark who was convicted of murdering her two children. The key evidence for the prosecution was an expert witness who said that the probability of two children in the same family dying of natural causes at such a young age was one in seventy-three million. Now, there was a lot wrong with that claim, but even taken at face value it ignored the fact that double homicides are just as rare as double accidents, and that in a country with more than sixty million people, you can expect to see one in seventy-three million chances come up every now and then.”

“A priori probability gets confused with a posteriori probability,” interjected the professor, apparently not willing to let being tied up keep him from giving a spontaneous maths lecture.

Loath as she was to agree with the man behind the whole nightmare they’d just been through, Liza nodded. “I’ll take your word for it. But the general point is that unlikely things do happen, and—and this isn’t proper maths, so don’t shout at me—if you’re an unusual person, you sort of make unusual things happen around you by definition. Like most people have never met Taylor Swift, and most people you meet will never have met Taylor Swift.” Why she was using Taylor Swift as an example, she couldn’t say. Was she blowing this? She hoped she wasn’t blowing this, but it turned out that summarising murders was way harder than it looked on TV. “But if I was Taylor Swift, then any room I was in would have a much higher ratio of people who’d met Taylor Swift than an ordinary room.”

The DCI’s expression had gone from indulgent to strained. “I hope you’re going somewhere with this, because it’s getting rambling, and I don’t like rambling.”

“The point is that organised crime does exist. And while most people never meet anybody who is involved with organised crime, once somebody who is involved with organised crime walks into the room, things change.” For a moment she let that hang, hoping that it could rescue the Taylor Swift analogy.