Sir Richard gave a snort. “A strange opinion both for a woman who has dedicated her career to the subject, and for somebody who has just spent several days in immediate danger of death.”

“Not boring like that. And obviously, I find it really interesting as a subject. But criminology—and before you say anything, DCI, yes, I’m not a criminologist—is just like any other subject: most of it is rubbish. Most art is bad. Most maths is just adding up—”

“It most certainly is not!” exclaimed the professor, seemingly more sincerely offended by this than by the accusation that he was a mass murderer.

Liza sighed. “I mean, when most people do it. I’m sure there’s some incredibly beautiful mathematics out there, just like there are incredibly beautiful paintings, and incredibly fascinating crimes; but in day-to-day life, most maths is splitting the bill, most paintings are signs outside pubs, and most crimes are shoving your husband off a balcony because you got in an argument on a snowy night.”

Ruby arched an eyebrow. “I don’t think that’s most crimes.”

“Not the details. The, you know, the spirit. Back me up on this one.” She turned to DCI McManus. “When somebody gets killed, it’s almost always their husband, or their wife, or a member of their family. And it’s always because they were angry, or jealous, or greedy.”

“I’m not sure I’d put it quite that simply,” said the DCI, “but I get what you mean. You’re far more likely to get shanked with a kitchen knife by your missus because she’s sick of the way you grind your teeth than to get poisoned with hemlock by your secret granddaughter’s secret lover because they think you stole your aunt’s emerald necklace.”

“For the record,”—Mary stepped forward again—”I don’t have a secret lover, I didn’t poison or stab anybody, and there’s no necklaces involved anywhere.”

Taking grudging vindication as better than no vindication, Liza nodded. “Yeah. That. I mean, the grinding-your-teeth that; not the emerald-necklace that. I think … this is really dull, but I think for at least half the murders that’s what we’re looking at.”

“Which half?” asked DCI McManus, suddenly taking Liza far more seriously than he had a few minutes ago. And it was a strange feeling, because the part of her that had grown up on detective stories and true crime had always wanted to be in this position, to be the one who worked it out. Only now she was here, she was realising it came with a whole lot of scrutiny.

“The way I look at it,” she began, “everybody except Belloc and Vivien Ackroyd. I don’t necessarily know who did all of the rest, but they’re the only ones I think have … where I think it gets conspiracy-ish.”