“What if . . . ,” I started. It was a crazy theory, but we were already talking about crazy things. “What if Grey unlocked the door? What if she was here yesterday? What if she left something for us, something only we would know how to find?”

Vivi snapped her fingers at me. “Now you’re thinking like a Hollow.”

* * *

We spent the afternoon combing Grey’s bedroom, excavating for signs of our sister. Vivi found the first: a collection of runes hand-embroidered in emerald thread on the underside of her mattress. I found the second—an annotated page torn from a book of Emily Dickinson poetry rolled up tight inside a curtain rod—and the third.

I was on my hands and knees under Grey’s bed, trailing my fingertips over the baseboard, when my skin caught on a splinter in the wood. A prick of blood welled on my fingertip. I wiped it away on my uniform skirt, then crouched to inspect the offending panel.

“There’s something here,” I told Vivi. A crack in the skirting board. I slipped my fingernails under the wood and lifted. It gave, but not enough. “Get me a knife from the kitchen.”

Vivi was standing over me now. “Why don’t you get it yourself?” she said.

I sighed and turned to give her a seriously? look.

“Sorry. Old sibling habits die hard.”

When Vivi returned with a butter knife, I slipped it into the crack, eased it open.

“You’re going to break it,” Vivi said as she tried to shoulder me out of the way. “Let me do it.”

I stopped and glared at her. “Would you quit it?”

“Fine. Whatever. Just be more gentle.”

“I am being gentle.”

Finally, the wood came away from the wall with a bright pop and clattered to the floorboards. Behind it was a hole in the plasterboard, its edges raw. The exact kind of place Grey liked to keep secrets. “Give me your phone,” I said to Vivi, and she did, the flashlight already turned on. I notched it into the darkness, eager to uncover more of Grey’s mysteries, the same way I had been as a child. My heart was clucking in my throat. The hole was laced with cobwebs. There were no diaries or cash or condoms or little bottles of elderflower gin, which is what we used to find in Grey’s hidey-holes before she left home. There was only a dried white flower, an antique hunting knife, and a brass key with a note attached.

“Now, this is more like Grey,” Vivi said as I handed her the strange treasures. I could sense Grey in them, feel her energy; she had touched them, not so long ago.

On the scroll attached to the key, there was an address in Shoreditch, followed by a message with yesterday’s date:

 Vivi, Iris,

 First of all, stop going through my private shit. I don’t know how many times I have to tell you. (Okay, fine, this time I was counting on you to go snooping—but still. Not cool.)

 I hate to say this, but if you’re reading this, I might already be dead. There is so much I wish I’d told you. Come to the address above. Bring the key. Find the door. Save me.

 And if he comes for you—run.

 I love you both more than anything.

 Grey

 P.S. Regarding the knife, as Jon and Arya said:

 Stick ’em with the pointy end.

8

I might already be dead.

We were in an Uber on our way to the address Grey had given us. The words from her letter were stinging through my nerves, pricking tears at the corners of my eyes. Vivi was silent, her forehead pressed against the window, her jaw set so tight I worried her teeth might crack.

I turned the knife over in my hands. Opened it, closed it again. The staghorn handle had a patina and the blade was pocked with age, but it looked slip-through-your-skin-like-butter sharp. It was the same knife we’d been found with as children, the one police had taken from Grey the night we came back. For a while they’d thought it was an important piece of evidence, but the only fingerprints they ever lifted off it belonged to my sister. I wondered how she’d retrieved it from evidence, considering our case was still open, then dropped back into the pool of my terrible imagination.

A world without Grey was impossible. Both of my sisters were the great loves of my life. I couldn’t live without them. I didn’t want to.

The Uber pulled up outside a pub in a narrow graffiti-lined street. It was late afternoon and there were a handful of Londoners huddled in the honey glow of the bar, drinking pints. The address Grey had given us was for the flat the next floor up. Vivi and I guessed the passcode to the building—162911, the same as all our phone passcodes—then walked up the winding staircase to the second floor. Grey’s door was beetle green, the handle brass to match the key.

Vivi put her hand on the paint and shook her head. “She isn’t here either.” I knew it too, I realized. I knew what Grey felt like, the way her energy settled in a room. Old memories were coming back to me. How I could follow the trails of her through our house when we were children, retrace the footsteps of where she’d been five, six, seven hours before. What books she’d read, which pieces of fruit she had picked up, inspected, put back. It wasn’t like I could see threads trailing behind her, or smell the scent of her skin. It was a sense of general rightness. Yes, she had been here. Yes, she had done this. I could do it with Vivi too, though that held less fascination for me. Grey was my obsession. Grey was who I wanted to be.

One long, lazy summer in France, the first one without our father, I spent two months living an hour behind Grey. She was fourteen and just coming to understand the exhilarating power of her beauty. I was ten. Too skinny, too tall, still shy and awkward. To me, back then, Grey was a goddess. She wore white flowing dresses and wreaths of wild lavender woven into her hair. I copied everything she did, living in the liminal world she left in her wake. I didn’t have to see her all day to know where she’d been, what she’d done. Sunbathing on the roof. A midday swim in the river. Nectarines and hard cheese for lunch. A kiss with a local boy in the pews of the medieval church (I improvised and used my hand—still, the priest was not pleased when he found me).