I ran up to the door to Yesterday, sidestepping Kieren's hunched body, and slammed the heavy barrier closed. But I knew it was futile. The door had been unlocked with the only key capable of doing so: a flattened penny crushed by the long-distance train as it barreled into the local station and created a momentary rift between the dimensions.

Once opened, only another such key could reseal it.

"How did you know?" I asked Kieren, who continued to watch me from the ground for a moment before standing to his full height, almost a foot taller than me. "How did you know about the key?"

He peered down at me now as if trying to make out a form emerging from shadows. "I learned how to open these doors when I was a freshman here," he said, his voice tentative, questioning. I let that sink in for a moment. In my reality, I had been the one to discover the power of the flattened coins. But I guess in Kieren's world, things had been different.

"And now I think it's your turn to talk," he said.

I felt flustered at being so close to Kieren. I had managed to keep my sanity over the last year, as much as possible anyway, by avoiding him completely. As he had graduated almost two years ago and didn't live in my neighborhood, that hadn't been too difficult.

But now he was so close I could feel his warm breath land on top of my head. I took a step back, trying to find my voice. But he took an equal step to meet me, and I found myself backed against the wall in the small chamber that held the three doors.

Kieren broke away from me then and put another flattened penny into the coin slot of Yesterday, sealing it back up again. He knew better than to leave any evidence of his visit or to create an opening for anyone else who might follow.

"What's your name?" he demanded.

"Marina."

"How do you know me?"

"We were friends once," I stammered, not sure how much I could possibly share without breaking the promise I had made myself: none of my friends could ever know what had happened. What if they tried to change something, alter the past? Robbie was safe now. I needed it to stay that way.

"We were never friends," Kieren said, his voice betraying no feeling at all, and the words hit me like a shower of ice.

"I don't know what to say."

Kieren glared at me for another moment before stepping back. "Wait a minute," he muttered. "Weren't you that kid Brady was talking to in the darkroom that day? Like two years ago?"

I gulped and nodded.

"You were the one who followed him to the train station and saw Piper leave."

"Yes," I admitted. It made sense that Kieren would remember that, for in this reality, Piper McMahon had still gotten on that train. She had still taken her DW parents through the portal, and they had later gone back on their own.

But seeing me confront Brady that day in the darkroom was probably the only memory he would have of me. Everything else had evaporated for him.

Kieren surprised me then by reaching into his pocket and pulling out a deck of playing cards. I couldn't help but inhale sharply when I saw them. They were a special deck we used to play with when we were kids. Every card had a picture of a different city on the back with a map of where in the world to find it. Robbie and Kieren and I had played with that deck so many times, I could still point to any place on a map from Albuquerque to Amsterdam.

"You've seen this before," he stated—a fact, not a question.

I could only nod in response.

"Did you put some sort of spell on this deck or something?" he asked.

I shook my head, not sure what he was talking about. "Like a witch?" I tried to joke, but it came out sounding angry.

"The deck . . . it's like it called out to me. I was asleep. One of those dreams where you know you're asleep, but you can't wake up. The deck of cards was . . . I don't even know. Vibrating. I knew there was something it was trying to tell me, but—"

"What did you see in the portal, Kieren?"

He flinched momentarily, hearing his name float off my tongue so easily, then turned to eye the Yesterday door again. Its faint yellow light was now beginning to fade behind the cracks at the top and bottom.

"I saw myself on the floor in my rec room. I was maybe eight, nine."

"You saw yourself or you were yourself?" I clarified, assuming he had just misspoken.

It seemed to surprise Kieren that I knew this particular fact about DW: when you go to the other side, if the other you is present, you become them.

"No. It doesn't work that way with little kids."

"What do you mean?"

"The molecular structure has to be identical. That's why you become your other self. But if you're a little kid or a baby or something, your structure is too different. So you can just watch it like a movie."

"Oh," I said, realizing something in the moment that I had long wondered about. Just over a year ago, when I had gone into Yesterday to undo the building of the lake portal, my brother, Robbie, was supposed to come with me. All my friends were supposed to come—Robbie, Kieren, and Robbie's new girlfriend, Piper. Then we were all supposed to return together when it was done, meaning we would all remember everything—we would all remember each other.

But at the last minute, my mother had stopped them, which was why I had had to go in alone. Inside the door, I had found myself in Portland a decade before, and the little-kid version of my brother had been there. I had long wondered what would have happened if Robbie had made it through the door with me. Would he have become his younger self? And if so, how the hell would I ever have gotten him out again?

I had resigned myself to the fact that it was for the best they hadn't made it. But with what Kieren was now telling me, I had to acknowledge that it wouldn't have been a problem.

The plan would have worked, I realized, both relieved and devastated at the thought. It really would have worked.

"You were there with me," Kieren continued.

"Hmm?" I asked, coming out of my reverie.

"You were there with me on the floor in my rec room. You were young. Like six. And there was another boy who looked like you."

"Robbie. He's . . . he's my brother."

Kieren paused and thought about the name, which of course meant nothing to him since in this plane Robbie had been raised thousands of miles away in Portland. Kieren eventually just shook his head. "I don't understand," he finally said.

I knew I had to somehow explain this to Kieren without telling him the truth. "It's a different plane is all. You know how DW is." I smiled, trying to sound casual. "I guess we were friends in another life."

Far in the distance, I heard the first school bell of the day chiming. I waited a few seconds for it to be over, then turned back to Kieren. "I have to go."

"That was the first time I went in." He stopped me with his voice. "That was two weeks ago."

"You've gone in more than once?"

"The second time, the three of us were playing Twister and laughing. We were laughing so hard I couldn't breathe. You fell on top of me."

I must've blushed, remembering how embarrassed I had been falling onto Kieren that day. God, I must have been nine. He had grabbed me and tickled my sides before I could stand, and his hands had been warm on my skin . . .

"And then I went in again this morning," he continued.

I held my breath. What did he see? How will I explain it?

"Your brother wasn't there this time."

"Okay," I said, calming myself as a wave of excitement turned my stomach to acid.

"We were older, teenagers. And so I did become my other self, I guess 'cause the molecular structure was similar enough by then."

Kieren walked back up to me, closer than two strangers should ever really stand.

"I was on the couch with you. Your hair was soaking wet. I guess you'd been in the rain. You were wearing my sweatshirt."

I knew which night he meant. It was the night I had remembered hundreds of times since: the night he'd first kissed me.

"And I was telling you it was going to be all right," he continued, close enough that I could feel heat emanating from his warm chest, almost brushing against me. "And I kissed the top of your head. And then your cheek."

I yearned to take his hand, to touch his chin, rougher now than it had been that night with a manly stubble he hadn't been capable of before.

"And then—"

"I really should get back," I suddenly stammered, knowing that I couldn't let him go any further. I would be too tempted to tell him everything in a moment. I had to think clearly. I had to protect Robbie.

"Wait—"

I turned to head up the spiral staircase. I could feel him grab at my backpack, but I didn't stop, even when I felt something snap off it. I kept running until I was back in the science room and then through the corridor leading up to the school.

It wasn't until I made it to my first-period trig class that I realized what Kieren had snagged off my backpack: my ID tag.

Would he come looking for me now? And if he did, how could I keep myself from telling him everything?

° ° °

Mr. Martel appeared to be having a terrible day. Gone were the bad jokes, the sparkle in his eye. He looked tired, unshaven, and pissed off.

"That's East Township for you," I overheard Angela Peirnot whispering behind me to Adrian Washington, whom she was now sitting next to, her foot casually draped over his. "This place ages teachers fast."

I looked over my shoulder in time to see Adrian rub a hand up her thigh. Apparently they were a thing now. I wondered if Christy knew. She had been really into Adrian for a while there.

My eyes drifted away from the new couple, landing on nothing in particular while the recurring thought that had been gnawing at me all day continued to swirl.

Kieren made a key. He knows about the keys.

Angela giggled behind me, whispering to Adrian to stop tickling her. But I tuned them out, my body shrinking in on itself. As I did so, a new question sprang into my brain.

Did he discover them himself? Or did someone show him?

"Marina O'Connell," Mr. Martel almost spat in my direction, causing me to whip my head back around to the front.

"Yes?" I asked, sensing I was in trouble but having no idea why.

"What is this?" he asked, holding up a loose-leaf sheet of paper I had turned in to a substitute before winter break.

"Um, my notes?"

"Your notes? Surely not your essay notes?"

"Yes," I offered meekly, starting to sense where he was going with this.

"One sheet of paper? Your five-part essay on Genghis Khan that accounts for over half your grade, and, as of a couple weeks ago, you had one sheet of paper done? Did you work on it over Christmas, at least?"

I cleared my throat, looking around for backup. There must be someone else who had also blown it off?

"Well, it was assigned by one sub, but then the next sub didn't seem to know about it, so . . ." I let the sentence trail off. I was only digging the hole deeper, I now realized.

"So, you figured you didn't have to do it?" Mr. Martel demanded.

"I wasn't sure." Again I looked around, but everyone had their faces buried in their hands or were covertly scrolling through contraband phones under their desks. Thanks a lot, gang.

"You realize this is due in a week?"

"I'm . . ." Think, Marina, think. "I'm still doing research. I mean, I've read a lot about him since then."

"Where?" he demanded.

Wikipedia. "The library."

Mr. Martel's unrelenting green eyes bored into me for another moment, then he abruptly turned away. I felt sweat forming under my armpits. I took deep breaths to calm myself—a technique I had taught myself in the past year. When I was younger, being embarrassed would always make me cry. But I was too old for that now. I had to swallow it down, one way or another.

"Break up into small groups and compare notes. I hope the rest of you are further along."

The desks immediately began shuffling around me, and I was grateful for the noise. I spent the rest of the period sitting silently next to Jackson Spartam and Holland Pfeffer, who luckily for me was such a loudmouth it didn't seem odd that I wasn't contributing anything.

I was relieved when the bell rang, and I grabbed my bag to beeline for the door.

"Miss O'Connell," I heard Mr. Martel say before I could get there. It was like being harpooned with a large hook—wanting desperately to escape but knowing I was trapped.

The rest of the class filed out in seconds, and I found myself alone in the room with Mr. Martel, who perched owl-like on the edge of his desk.

"Please tell me you're going to rectify this situation."

"I am."

"Good."

He looked down for a moment, seeming somehow about ten years older than he had looked the previous day. Maybe it was the five-o'clock shadow or the crumpled shirt. What the hell had happened to this guy in the last twenty-four hours?

"Is that all?"

He cleared his throat as if coming back into the room from a faraway thought. "Yeah," he finally said.

I started to back away, grateful to get the hell out of there. Kieren's face was flashing before me, and all I could think about was what had happened that morning. Had it been real? I had imagined talking to him again so many times. It was hard to believe—

But before I could finish the thought, Mr. Martel stood up from his desk, and everything fell out of his pocket. "Shit," he muttered to himself.

I almost turned to leave, but I didn't want to be rude to the guy who held my GPA in his hands.

"Here," I offered, crouching down to help him pick up his dropped change.

He patted his empty pocket and seemed to remember suddenly what had been in there. Then he panicked, reaching out to stop my hand from retrieving what had fallen. "Wait, don't! Don't!"

I flinched away from him, something sharp cutting into my palm. I cried out from the pain and opened my hand to see what it was.

My eyes grew wide, and I stared up at Mr. Martel. He looked back at me, sheer horror in his expression. We were at an impasse, frozen, each waiting for the other to move as we crouched down together on the floor.

The bell rang, and several students began filing in. Mr. Martel stood, and I did the same. His eyes were plastered to mine, terrified once he realized that I had really seen it, that it was too late to pretend it was nothing.

I handed back what was in my hand, feeling how it had gouged a slash into my skin. I didn't even have to look at it again to be sure of what it was. I would know the feel of it anywhere. After all, I'd carried one in my pocket for months before I'd understood the power it possessed.

It was a penny, flattened by a passing train.

And the look that passed now between Mr. Martel's anxious eyes and mine told me one thing for certain: we both knew exactly what that meant.