Alex grandfather sat tapping a thumbnail against his teeth. “Please go on. I don’t see the connection with the paintings.”

Metz exchanged another look with Kingdom. She nodded.

“In the years following its destruction,” Metz continued, “many legends sprang up about the ruin of the destroyed castle in the forest at Boll: that some great prize lay buried in there. And so the place became a target for treasure hunters across the sixteenth and seventeenth centuries. For many years, the first incarnation of our society worked to keep curious eyes away—while simultaneously working among the ruins to conceal all traces of what had happened.

“In the early 1600s, they finally removed something from the site and hid it safely elsewhere. And to prevent this knowledge from falling into the wrong hands—what it was, where it was taken—the society scattered the information. They hid the key to it all in the paintings, which were then spread across Europe: those you already know about; this painting here above us; and—”

“And one other,” Kingdom quickly cut in.

“Another?” Alex and his grandfather spoke in unison.

“Yes.” She regarded them carefully, not quite masking her relief at their surprise. “One more. Put all the paintings together, and, the ancient secret can be known again.” She said the last words in a singsong, like a poem she had learned by heart.

“Something was removed.” Alex’s grandfather said, catching Alex’s eye. “So, ah, what was that? You’ve trusted us with this much, you may as well tell us the rest.”

“That’s the problem.” Kingdom sighed. She hung her head, then smiled ruefully up through a curtain of hair. “It’s not a matter of trusting you. We can’t tell you much more because we don’t know much more. You see, around the time the society became known as the Fishing Club, its leadership came to a great decision: that the best way to protect our secret was to let it be forgotten.

“The society always guarded its knowledge religiously—very few outsiders were ever told this story. But in the 1880s it was suspected that one member of the club had betrayed us. Nothing was proved, but it was thought he had been persuaded to reveal the secret to someone: some outsider. And so the Fishing Club built their hotel out there at Bad Boll at that point, as a way of increasing protection of the ruins: they could run regular patrols, even light the forest by night, without anyone wondering why.