“This is quite a collection of suppositions,” he said, when James was done. “But I must ask, especially you, Lucie and James—why do you not seek your parents’ assistance with this? Why the secrecy?”

“Because we made a promise,” said Matthew. “To the person who gave us the key that unlocked a great deal of this information. And we can’t break it.”

Magnus smiled an odd sideways smile. “Ragnor told me that he trusted you with some information, and it appears that you have not betrayed his trust. Not many Shadowhunters would honor such a vow made to a Downworlder. Since I am Ragnor’s best friend, or at least the only person who can tolerate him for long stretches, I will keep your secret.” He glanced from James to Lucie. “In days past, when I knew your parents well, they probably would have been spearheading this plan.” He stood up. “But now they are no longer children. They are parents, and thus devoted to something they love more than their own lives. So indeed, perhaps they should not be told.”

Even Matthew had no response to that.

“Well, good luck,” Magnus said, picking up his walking stick. “I suppose I shall go to Hatchards for a few hours. There is no better distraction in this world than losing oneself in books for a while.”

Cordelia stepped forward, her hands held out as if to prevent him from leaving. “Mr. Bane,” she said. “I know it is a great deal to ask, especially when you have promised to keep our secrets. But would you help us?”

Magnus tapped gloved fingers on the head of his cane. “You’re a Carstairs, right? Cordelia Carstairs?”

“Yes, I’m Jem’s cousin,” Cordelia said. “Look—we know this is a wild plan, but it could save many lives. You needn’t help us directly, or get involved in the fighting. I understand you feel loyalty to our parents. But you could help us very much just by casting a spell to keep mundanes off Tower Bridge while we venture onto it. It would be safer for them, too.”

Magnus hesitated. It was utterly silent in the Sanctuary. Cordelia imagined she could hear the sound of the blood pounding in her ears as Magnus considered her request.

At last the warlock shrugged a silk-suited shoulder. “Very well,” he said. “Even though that green bastard Ragnor has hared off to Capri, I don’t think he would have wanted you to put yourselves in danger because of a promise to him. I’ll keep an eye on you, but remember—if I see anything I think Will and Tessa need to know, I will tell them about it posthaste.”