Now that I had found the ingots, I crossed to examine them and introduced myself, as any lead mine slave would, by giving them a friendly kick. I hurt my toe, but did not care; at least I knew for certain this ghostly mass was real. As I bent to rub my foot my hand hit a small object hidden against the leaden stack. I held it up: it was a plain brass inkwell, its contents long ago dried up. All three of us looked at it but none of us spoke. I put it in the pocket of my tunic slowly, then shivered in my holiday cloak.

Helena Justina spoke up with a hint of dramatic urgency: “You are trespassing, Falco. I want you to go.”

I turned. As our eyes met, I felt the sudden familiar lift in my spirits. I felt certain, too, that we were partners sharing a charade.

Now there were three of us in the vault, a new tension had taken effect. It felt like belonging to a geometric problem where certain fixed elements would enable us to draw the figure if we followed Euclid’s rules. I smiled at her ladyship.

“I finally worked out that a few barrels of nutmeg were not enough to keep bringing down the roof of the Cloaca Maxima. Lead bars would though! The political plot has foundered; so the ringleader probably intends the ingots for himself. I’ve also worked out that he’ll make for the pigs and then make off. There’s a neat row of heavy-duty waggons in the yard that I reckon are due to leave laden with silver after curfew tonight. When he comes for them, here I am.”

“Falco!” cried Helena, apparently in outrage. “It’s my father—you cannot arrest papa!”

“Titus could. Still,” I commented drily, “in cases of treason we spare senators the inconvenience of a public trial. His honour can expect to receive a warning note in time to fall tidily on his own sword in the privacy of his very select home—”

“There is no evidence,” Helena argued.

Sadly I disagreed. “A great deal of circumstantial evidence has always pointed direct to Decimus. From his first volunteering to assist his friend the praetor, through to the way you and I were ambushed, and on to an unsavoury man who was planted in my rooms during the period when your father was so conveniently paying my rent…As a matter of interest, ladyship, why have you never mentioned the existence of this vault? What are you intending to do—let your father make good his escape with what silver there is? Very loyal. I’m certainly impressed!” She stayed silent, so I turned to her uncle, still playing the ingenuous part. “Bit of a turn up for you, sir? Your highly placed brother named as Domitian’s paymaster—”

“Shut up, Falco,” Helena said, but I went on: