I tried to reason with him. “You’re like one of those people who’s more scared of flying than driving, even though you’re way more likely to die in a car.”

“Yeah, but if I’m in my 1981 Pontiac, at least I’m going out in style,” he said, and I laughed.

Paul, from his place at the far side of the great room with my parents, shot me a look—not jealous, but hopeful. He wants things back to normal with Theo. That has always meant laughter.

The two of them have always been such good friends. They seem to have so little in common besides their interest in physics: Paul in his plain secondhand clothes, clueless about pop culture, while Theo wears fedoras and Mumford & Sons T-shirts. Yet they’re both young for doctoral students: Theo is twenty-two, and Paul just turned twenty. They both believed in Mom and Dad’s Firebird project when few others did. And they became a part of our weird little family. During that time—while everyone was involved with rewriting the laws of the natural universe as we knew them—emotions got confused.

(“We ought to have expected it,” my mother said the first time I talked to my parents about all this. “Isolating individuals for long periods of time, away from any other likely romantic partners, particularly at this highly active stage of sexual development—strong emotional bonds were all but inevitable.”

“We don’t care about each other only because we spent so much time together!” I protested.

“Of course not, sweetheart.” Dad patted my hand. “Still, you have to admit it helped.”)

Both Paul and Theo fell in love with me. I fell in love with Paul.

It’s not that I don’t care about Theo; I do, deeply. I’d be lying if I said I hadn’t been attracted to him sometimes. For a brief time—when Paul had been framed for my father’s murder, and I was sick with grief and betrayal—I wondered whether Theo wasn’t the one for me after all.

But Paul and I came back to each other. And Theo was left on the outside looking in. Even though all three of us know nobody did anything wrong in this scenario, both Paul and I haven’t been able to help feeling awkward when Theo sees us together.

That night, however, I could almost believe nothing had changed. Our house in the Berkeley Hills looked exactly the same, with houseplants in every corner and on every shelf; the hallway black with chalkboard paint and thickly covered with equations; the rainbow table exactly where it should be; and stacks of books nearly as high as the furniture around us. Paul’s plain backpack and Theo’s battered messenger bag were nestled by the door along with my denim jacket and Mom’s bike helmet. The guys still practically live with us, like most of my parents’ grad assistants over the years.

Yet Paul and Theo have always been different from all the rest. Closer to us, more important. I knew that even before they finished assembling the first Firebird.

“Thought Josie was going to get here tonight,” Theo said. “Wasn’t she coming in from San Diego?”

“No, she said the surfing was too great today to pass up.” I squirted a little more dishwashing liquid into the sink. My rubber gloves were bright pink; Theo doesn’t bother with them, so suds covered his wet hands. The bubbles smelled like lemons. “She’ll fly in tomorrow.”

Theo shook his head. “If the waves are that good, I’m surprised she’s coming at all. Not like Josie to pass up a chance to surf.”

“After what happened to Dad, or what we thought happened to Dad . . .” I didn’t have to finish the sentence; the look Theo gave me told me he got it. My family has always been tight-knit, but now that the world’s turned against us—and we know what it would mean to lose each other—it’s like we can’t be close enough. With a smile I said, “So that’s her spring break. What about yours? Doing anything fun?”

“I learned my lesson,” he said. Last year Theo dragged Paul to Vegas. Paul got thrown out of Caesar’s Palace for counting cards, because he didn’t understand that casinos consider “mastering probability theory” the same thing as “cheating.” He got to keep his winnings, but apparently he had to spend all that cash buying back Theo’s muscle car, which he’d wagered on a losing round of baccarat. They came home better friends than ever, but Paul said he didn’t see the point of spring break.

Theo would always see the point of a party. But no matter how casual he acted about hanging around here, he wanted to stay close too.

Still, there are other ways to travel. . . .

I had meant to ask this for a while, but that night I finally felt comfortable enough with Theo to speak. “When are you going to take a trip of your own? See some other dimensions for yourself?”

He was quiet as he placed another plate in the dishwasher. Finally he said, “I don’t know that I ever will.”

“Not ever?” During the past two years, Theo had been more psyched than anyone else at the thought of seeing new worlds.

He turned to me, and I’m not sure I’ve ever seen his expression so serious. “I’ve seen what it’s like from the other side, Marguerite. That part isn’t as much fun.”

Last fall and winter, whenever we talked to Theo, whenever he persuaded us to do something—we weren’t talking to our Theo. It was his body, but the consciousness inside belonged to the Theo Beck from another universe—acting as Conley’s ally and his spy. That Theo was the one who arranged my father’s kidnapping and framed Paul for his murder. The one who turned me into Conley’s ideal “perfect traveler,” then persuaded me to take my first voyages with the Firebird.

The one I clung to after I thought my dad was dead—the one I kissed in another dimension, and at my weakest moment nearly slept with—and the one Theo blames for destroying whatever the two of us might have had.

Theo’s wrong about that. For me it was always Paul; it could never have been anyone else. But the other Theo’s shadow hangs heavy between us.

“I still crave it sometimes, you know.” Theo stared out the kitchen window into the darkness beyond. “The Nightthief.”

Nightthief is the one and only way to cheat the rules of traveling between universes. It’s a drug—emerald-green, injectable, invented in the Triadverse—that allows a dimensional traveler to maintain control. See, a traveler on Nightthief remains as much in control as I am. But the drug has certain serious drawbacks. One, it’s addictive and can cause seizures. Two, Nightthief can be made out of what we’d consider common household chemicals—but if you’re in a universe where those aren’t common, you won’t be able to supply yourself. (While consciousness can travel through dimensions easily, it’s very, very hard for physical matter to travel. So forget bringing any Nightthief with you.) Three, the drug wears off after a day or so, which means if you don’t have more on hand, you’re screwed.

When the other Theo took ours over, he took that drug for months on end. My Theo—the one who stood beside me that night, our elbows brushing—he had to go through the withdrawal. Worse, he had to live with the memories of another Theo using his body to endanger and betray us all.

“It’s not always like that,” I said quietly. “Paul and I take care of our other selves. We try to live their lives, as much as we can. We’d never make them do anything they wouldn’t want to do on their own.”