Detective Rule: In moments of severe distress and danger, a detective’s most valuable possession becomes a very good hiding spot.

MATTHEW LEIGHTON HELD HIS INDEX FINGER to his lips in a gesture for us to hush. My jaw, and Adele’s, hung open. Neither of us made a sound as he came inside the study and closed the door lightly behind him. But once he’d closed us off from the hallway, Adele straightened her back and the questions began.

“Who are you? What are you doing inside my house?” Her voice trembled, but she still jutted out her chin commandingly.

“You must forgive me, but I’m not accustomed to explaining to the resident of a home why I’ve broken in.” He took his lit match to the desk and swept it over the lantern wick. The study brightened. He blew out the match. A gray tendril of smoke drifted in front of my grandfather’s face. “But Suzanna can assure you I’m quite harmless.”

Adele swung her shocked expression toward me. “You know him?”

I paused, choosing to look at him instead of Adele. “He’s Matthew Leighton.”

“The one your uncle is trying to arrest!” she exclaimed. “But this doesn’t make any sense. Why can you assure me he’s harmless?”

Leighton remained silent. He simply looked over to me, waiting to see how I would respond. He was giving me the choice, I realized. The choice to lie or to admit the truth. I wasn’t going to lie.

I lifted my chin. “He’s my grandfather.”

The corner of Leighton’s mouth twitched up with surprise. Adele gaped at me.

“Your … grandfather?” Adele’s gray eyes narrowed into slits. “Your grandfather is the thief and you knew it? That’s what you were keeping from me?”

“No! I mean, yes, I knew, but —” I saw the fury boiling up fast in her widened eyes. “He is not the thief, Adele! And he’s certainly not an arsonist or murderer.”

But I couldn’t explain why he was inside her father’s mansion. I turned toward him. “What are you doing here anyway?”

He walked out from behind the desk, still wearing his hat and coat, and even his gloves. It didn’t look like he was planning to stay long.

“There isn’t a lot of time to explain, Suzanna. I expect your uncle to be here very soon.”

Adele and I glanced at each other as my grandfather moved to the floor-to-ceiling wall of shelves. Books packed every shelf.

“Why would he be coming here?” I asked. He inspected the titles, not turning around to look at me when he replied.

“I fear you’ve too quickly dismissed your uncle’s competence. You know as well as I do how distasteful the idea of failing is to him. He failed apprehending me on the Common yesterday. Did you consider he might have a backup plan?”

He ran his hands along a row of leather spines — I wanted to know what he was searching the shelves for.

“But Uncle Bruce is supposed to be at the bon voyage dinner,” I said as my grandfather came to the end of the shelves and to the stones of a cold, fireless hearth.

“Yes, as are you,” my grandfather said, glancing over at Adele. “And you. But here the two of you are. Inside a house that was supposed to be empty all evening. Once again thwarting your uncle’s plans.”

Of course. Uncle Bruce had been planning on leaving the dinner party early, and then staking out the Horne house. For some reason, he believed Leighton would show up to take advantage of the empty house. But why? And how had Leighton known?

“If you know he’s coming, why are you here? You’re walking into his trap,” I said.

Leighton bent down and peered inside the hearth. He stretched his arm inside and felt along the blackened inner walls of the fireplace.

“It’s not his trap. It’s mine. You see, a concerned neighbor sent in a complaint to the police station saying an old man had been seen prowling around the Horne house the last few nights. If I know Bruce’s modus operandi — and I do, very well — I know that he will attend about an hour of this bon voyage dinner out of courtesy before slipping off to see if he might catch me in the act. He wants to trap the Horne art thief, and trap the thief he shall.” He glanced back at me and propped up one eyebrow. “It just won’t be the thief he’s expecting.”

As my grandfather ducked and entered the deep concave of the hearth, I couldn’t help but marvel at his astute, clear thinking. He would have made a brilliant detective — that is, if he hadn’t already been a thief.

“What are you doing?” Adele asked as Leighton’s shoes scuffed through the old, cold debris on the hearth’s stone floor. They brought up clouds of ash. There came an audible click, and then an entire panel of books beside the hearth — six shelves running at least six feet long — popped open like a door.

Adele stared in disbelief at the open panel. My grandfather reappeared, dusting off his shoulders and hat. He grabbed hold of the open panel door to what, I now realized, was one of Mr. Horne’s hidden safes.

“How did you know about this safe?” Adele asked. From her dumbfounded expression, I deduced she had not.

Grandfather grinned and gestured to the wood floor. The bottom of the door ran flush along the floor, and now, looking closer, I saw an arc of scratches along the polished wood from the many times Mr. Horne must have opened the safe.

“I’ve taken a few unguided tours of your home before, Miss Horne.” At Adele’s widened eyes, Grandfather continued, “Don’t be alarmed. I left everything in its original place.”

I took another look at the scratched floors. My grandfather had the eyes of a detective as well. Perhaps that’s what made him such a good thief.

Before I could look back up, my eyes traveled over the tips of Grandfather’s black dress shoes. A silky layer of ash coated the shiny patent leather. Mr. Horne had come to Grandmother’s dinner party with his shoes dusted in just the same way. He’d been taking care of some business, he’d told my uncle, but had avoided saying what about.

He’d also been elusive about something else: his collection’s crown jewel. I was willing to bet it was right inside this secret safe. Mr. Horne must have gone to wherever it was being stored, extracted it, and moved it to his own home without telling a soul. All the while Detectives Snow and Grogan had been at the “club” waiting for him, as I recalled my uncle saying.

“Miss Horne.” Leighton checked the hands of the mantel clock. Five minutes to eight. “It would be best if you and my granddaughter left the house now. Suzanna, hire a hackney back to your grandmother’s house and stay there until everything your uncle and I have planned for tonight — separately, might I add — has unfolded. Quickly, girls.”

“Don’t!” she cried as Grandfather started to duck inside the safe. I was certain she’d figured out what was inside as well.

He stopped, but not because of Adele’s plea. The floorboards had creaked in the hallway outside the study once again.

“He’s arrived early,” my grandfather whispered.

“Uncle Bruce?” I crossed the room and peered out the window. The rain had tapered off and I could see easier through the streaked glass. There was no one out front, though the statues of Hercules and the armless goddess made me take a second glance.

“No,” my grandfather answered. “The real thief.”

He’d followed me to the window and startled me now by shoving me behind the floor-length curtains.

“Make yourself invisible, Suzanna,” he whispered, pulling the curtains farther along the rod to obscure me. He then hissed to Adele, “Miss Horne, hurry. Over here —”

Before he had the chance to finish, the study door squealed on its hinges.

Adele screamed yet again. I rolled my eyes, recalling my Detective Rule against that particular reaction.

“You! But you’re … you’re …” Adele stuttered.

I widened my eyes, though the only thing to see was the back of a thick panel of green velvet. But then the next voice made me want to scream.

“Dead. Yes, I know.”

It was Detective Grogan. He was standing inside Mr. Horne’s study. And he was totally and utterly alive.