“ ‘Low the swan flies,’ ” Sally said, reading the first line. “What does it mean?”

“It’s not talking about a bird. ‘Swan’ is a reference to a substance apothecaries use. It’s called that because, just as a cygnet changes into a swan, this substance can change its appearance completely.”

And there it was.

I turned Master Benedict’s notes around so they could read it.

Tom scanned the page until he spotted the right symbol. “ ‘Arsenicum album,’ ” he read. Then he saw the translation next to it. “But that’s…”

I nodded. “White arsenic. This message is a warning. Someone’s about to be poisoned.”

CHAPTER 32

TOM WAS HORRIFIED. “WHO?”

“That’s what I don’t know yet,” I said. “The answer has to be in these other two symbols. Or the riddle.”

“They’re not alchemical?”

“I don’t think so. Alchemical symbols are all lines and swirls. These look like they mean something else.”

Sally leaned over the message, frowning. Then she looked up, surprised. “Berkshire House.”

“What?”

“This.” She pointed to the third symbol. It looked like the outline of a mountain, with three diamonds. “I saw it on a banner at Berkshire House.”

“Are you sure?”

“Absolutely. It’s hanging over the mantel, in the parlor.”

“So that means… someone will be poisoned at Berkshire House?” Tom said. “But who?”

Whoever the oak leaf referred to, I would guess. “Oak symbolizes a lot of things,” I said. “Strength. Long life. Faith. Loyalty. The oak is the king of trees—”

Tom gasped.

“The king!” he said, eyes wide. “What if the oak is the king?”

“Why would it be?” Isaac said.

“Because of the oak where His Majesty hid, to avoid Cromwell’s troops. It’s why we have Oak Apple Day.”

It wasn’t surprising that Tom, wearing his hat again, would think of the king first. But there was a certain logic to what he said. What’s more, if the Covenanters were our enemy, then they were definitely after Charles. We’d just have to figure out how they’d get him.

Turns out Sally already knew. “Tom’s right,” she said. “It is the king. They’re going to try to poison him. And they’re going to do it tomorrow.”

“How you do know?” I said.

“The riddle. ‘Slow the song plays, ever slower, dancing, singing overhead.’ What does that sound like?”

“A party,” Tom said.

“Remember what I told you when you came to get me? I’ve been invited to a party. It’s tomorrow—at Berkshire House.” She tapped the last symbol on the letter. “The king’s supposed to attend.”

Tom rose. “We have to call it off!”

“No,” Isaac said suddenly. “Don’t.”

We stared at him, puzzled. “But… the king!”

“Warn him, yes. But look.” With a slight grimace of pain, Isaac leaned forward. “If you cancel the party, you will prevent this poisoning. What about the next one? How will you stop that?

“This message puts you ahead of the Covenanters, do you see? If you say nothing for now, they won’t know their plot has been discovered. They’ll continue on as usual. So if you use the time to flush them out—”