Once Mitzi and Noodle had left the room, Tom and Colby wasted no time squeezing themselves between the two towers of stacked glass cages. In one cage, a diamond-backed snake reared back, its tiny fangs bared at Colby. She instinctively moved a little closer to Tom and shuddered. Colby was on overload, trying hard to be nonphobic in the face of all these germy animals.

“On three. Lift and slide.” As Tom’s fingertips hooked under two corners of a bottom cage, readying himself, three salamanders scurried to its other end and stared up at him with black, unblinking eyes.

“One … two … three …” The metal screeched against the wood.

“She mighta heard that,” said Colby, cutting her eyes toward the door.

They waited, breath held, for Mitzi and Yoo-Hoo to come storming into the room, but there was no sign of them. Tom and Colby were in the clear. For now.

Tom turned toward the now exposed brick wall behind the stack of cages. At first glance, it didn’t look too promising, so he ran his hand along its rough surface, his fingers searching the braille-like grooves for some sort of clue. And then he felt something.

Near the bottom of the wall, in the middle of the brick, was a small indentation and a smooth patch no bigger than a quarter. He knelt, his face inches from the floor, and came eye to eye with an encircled rose imprint etched into the brick.

“Colb!” he whispered loudly. “Over here. It’s the seal of the Sub Rosa!”

“Shut up!” Her jaw was on the floor as she huddled in close next to Tom. “Up till now, this all seemed too farfetched … but …” She reached her hand out to touch the sanded grooves of the rose petals.

The mortar surrounding the brick was a different texture, Tom realized. Lighter, too, as if it had been more recently replaced. Something must be hidden behind that brick.

“How can we get to it?” Tom wondered aloud. “Ugh. I never have C-four explosives when I need them.”

“That’s one sentence you are never, ever allowed to say in the presence of my nana.”

Tom leaned against the wall and wracked his brain. Mitzi would be back soon—even Noodle couldn’t keep her occupied forever—and there was no way to get through that brick. In a week and a half, he’d be in Kansas and might never get an opportunity like this again. If he could just get his hands on a strong acid.

Vinegar’s acidic, Tom thought. But not enough, unless I combine it with something corrosive.

Chemical combinations and reactions swirled through his mind at rapid-fire speed, years and years of basement experiments coming back to him.

After a moment, Tom popped to his feet, snapping his fingers.

“All right, Colb,” he said, turning to her, his face the picture of focus. “We need to split up.”

“Uh-uh, Tom. I know that look. What’s going on inside that brain of yours?”

“I can’t explain right now, but chemically speaking, this should work.”

“On second thought, I don’t even wanna know what—”

“I know you don’t. But we don’t have time. You need to go to the bathroom, get us some powdered bleach, a plastic bucket if you can find one, and a whole buncha paper towels. And whatever you do, don’t let Mitzi see you.”

Colby was about to rattle off the top twenty reasons why whatever Tom was thinking about doing was a terrible idea, but something stopped her. Maybe it was the pleading look in his eyes. Maybe it was the excitement about what could be behind that wall. Or maybe Colby McCracken had become so used to the sneaking and risk taking of this odd treasure hunt that she’d grown a teeny bit braver these past few days. Whatever the reason, she found herself nodding her head.

“Okay, I’ll do it. What about you?”

Tom hesitated, surprised by her response, then smiled, with eyes glimmering mischievously. “I’m going through that window. We’ll meet back in five, so you can hoist me up.”

“Cool.”

Placing trust in her friend when safe logic and sound reasoning had failed her, Colby disappeared into the hallway.