“Thanks.” Josie ruffles my hair in the way she knows I hate. Normally that’s my cue to scowl at her, but right now I even love her messing with me.

The last time I saw Josie, she was sobbing hysterically in Mom’s arms. Now she’s her usual laid-back, beach-girl self, complete with flip-flops and a sunburned stripe across her nose. As I study her face, I recognize anew all the ways in which she’s similar to my father: the blue eyes, the square jaw, the chestnut color to her hair. I’m the one who looks like Mom, more like Vladimir and Peter—

That stops me short. Only now do I remember I’m in a world where my brothers and little sister never existed.

“Are you okay?” Josie gives me a funny look. Behind us, I can hear Theo putting the lasagna in the oven.

“Yeah. I’m good. It’s just—” I make a fluttery gesture with one hand, which is supposed to mean something like, I haven’t got my act together right now.

But Josie’s expression hardens, and I realize she thinks I’m talking about Paul, and the scars his betrayal have left on the family. That’s why she’s home for New Year’s instead of partying with her friends; she’s trying to help our parents get through it.

“Mom and Dad are in the great room?” Josie asks, dumping her backpack at the door like she has ever since fourth grade. As she lopes in to see our parents, I lean back against the fridge, disquieted.

When Theo gives me an inquisitive glance, I motion toward the great room. “Go on, hang out for a while. I need a second.”

He doesn’t look 100 percent satisfied with that response, but he nods, giving me the space I need.

After Theo leaves the kitchen, I stand there staring out the kitchen window. (At home, we have a suncatcher dangling there, a little orange and yellow butterfly. Here, the suncatcher is in the shape of a bird, all blue and green.) My heart aches, and this time, there’s no cure for it.

I can see the irony. Throughout this journey, I’ve longed to be with my family again. Now I’m with them, more or less, but I have another family to miss.

Katya and little Peter—I never even got to see them after the attack on the royal train. Peter must have been utterly terrified. He won’t be able to sleep at night; I ought to have a couch brought into my room for him, so he can rest nearby, so I can wake him if he has nightmares. And Katya? Probably she’s already arguing that the tsar should allow women in the army. And Vladimir will be urging the tsar to consider more constitutional reforms, so that no other pretender to the throne can rise up to capitalize on the dissent . . .

I should be there, I think, before remembering that, of course, I am. The Marguerite who belongs in that dimension is back in charge of her own life. We are enough alike for me to know that she’s taking care of Peter, and that she’s adding her voice to Vladimir’s, for whatever it will be worth with the obstinate Tsar Alexander.

She’s also mourning the loss of Paul Markov, her Paul, dead and gone forever.

Does she even remember her final weeks with him? Does she know that she was able to spend one night with Paul, one night when all the barriers between them came down? If not, then . . . I stole that from her. Something sacred that ought to have been hers alone became mine forever.

I told Theo earlier that I didn’t think Paul was the villain here.

Now I realize the villain might be me.

“So, I was wondering about the ethics of traveling through different dimensions,” I say at dinner.

Mom and Dad exchange glances, and Theo gives me a look like, Are you crazy? I pretend I don’t see him.

“We’ve had these conversations often enough,” Mom replies as she helps herself to a piece of the lasagna. “Forgive me, sweetheart, but I never thought you were interested.”

I have to admit this is more true than not. If I didn’t tune out some of the heavy-duty physics talk from time to time, I’d go crazy. Besides, when was any of this theoretical stuff going to apply to my real life?

Now, of course, I know the answer to that question.

“When you guys were talking about it before, it was always, you know, ‘what if.’ Abstract, not concrete.” Hopefully I sound casual, just interested enough to make conversation. “Things have changed now.”

“Yes, they have,” Dad says heavily, and I know he’s thinking of Paul.

We are gathered around the rainbow table, temporarily cleared of its papers to make room for lasagna, salad, garlic bread, wine, and a ceramic pitcher filled with ice water. (The Nobel Prize is on the floor beside a stack of books, all but forgotten.) In so many ways, this scene is exactly the way it ought to be, cozy and shabby and unmistakably ours. Mom’s hair is pinned back into a messy ponytail with two pencils. Dad wears reading glasses with rectangular, tortoiseshell frames. Josie smells like cocoa butter. Theo has his elbows on the table. And I’m kicking the center pedestal of the table, a nervous habit my parents gave up trying to break me of when I was in junior high. There’s even a package of shiny hats Josie brought, like she does every year, though we won’t put them on until nearly midnight.

Yet there’s an empty chair at the table, a place where Paul should be and isn’t. The most powerful presence in the room is his absence.

“We thought it would be a chance to glimpse a few small layers of the multiverse though another set of eyes . . . and then we would return home, to share the knowledge.” Her gaze turns dark. “But apparently knowledge isn’t enough for some people.”