Josie caught sight of her mom sitting on the edge of Jo's bed. "It worked out."

"I suppose I could have told you," Jo continued, speaking in a very calm, deliberate tone. "About our moms, and about what my father was." She looked up at Josie. "He told me one day that he'd tried to kill my mom. And if I didn't do exactly as he said, I'd be next. He was deranged."

"I'm so sorry, Jo."

Jo shrugged, and her carefree attitude immediately returned. "Oh well, he's dead. Too bad, so sad."

Jo may have been totally unfazed by her father's gruesome death, but the thought of Mr. Byrne's mangled body made Josie shiver. As happy as she was that he was gone, it was a death that would haunt her for the rest of her life.

Jo glanced at Nick, then back to Josie. "So you're really going back?"

Josie's stomach clenched. The time was rapidly approaching when she'd have to make a decision: stay or go. She honestly wasn't sure which it would be. "I think so."

Jo stared at her for a moment, assessing; then her eyes drifted down to Dr. Byrne's necklace that still hung around Josie's neck.

"Oh," Josie said. "Right. You probably want this back." She reached up and placed her hand on the necklace, two entwined hearts. Josie and Nick. He hadn't given it to her, but somehow it had come to symbolize all that they'd been through together, and the idea of taking it off was almost as painful as the idea of leaving Nick forever.

Jo continued to stare at the necklace. "You really thought Nick gave that to me."

Josie nodded.

"You're in love with him, aren't you?"

Josie nodded again. "Yeah."

Jo's face was blank and impassive. "But you were going to give him up. Back at Fort Meade, you said you'd save him for me."

"Yeah." She had said that. And she'd meant it, realizing at the time that the odds of her and Nick actually being together were very slim.

"You know, he's the only guy that ever said 'no' to me. I think that's what made me want him so badly." Jo pointed to the necklace. "You keep it. As if he gave it to you." Then she turned on her heel and walked to the window.

Josie's mom sat on Jo's bed staring at the mirror. "I can't believe it's all come down to that wretched mirror."

Josie sat down next to her and squeezed her hand. "Think of it this way: if the initial explosion hadn't established a connection between the two mirrors, I never would have known you were gone, never would have been able to come here and find you."

Her mom sighed. "That's one way of looking at it."

"Personally," Nick said with a smile, "I kinda love that mirror."

Josie looked at Nick. Despite his smile, his eyes were sad. Her heart ached at what was coming, and when Nick twitched his head toward the hallway, Josie stood up and followed.

"Stay," Nick said as soon as the bedroom door closed behind them. "Stay here with me."

"I can't. My mom needs me."

"I need you."

Josie swallowed. "Come with me, then."

Nick reached up and cupped her cheek with his hand. "But my brother . . ."

"I know." Josie dropped her eyes. She leaned forward and rested her head against his chest, listening to the rapid beat of his heart. Could she stay? Her mom was going home to her dad, and hopefully they'd be able to reconcile once Dad knew the whole story. With Tony completing the Nox injectable and Dr. Byrne there to oversee the eradication, this universe would be safer. She could send her mom home with the formula to help do the same in her world. And besides, maybe she could contribute something here, explore the physics of this universe with a different perspective. It was an enticing possibility.

But the only thing that really mattered to her was Nick.

"Okay."

They said it in unison, both Josie and Nick. Josie looked up and Nick's head was tilted to the side. "Did you say 'okay'?"

"Yeah." Josie laughed. "Did you?"

"Yeah."

That was it. They needed each other, were desperate to be together no matter whose universe they chose. Josie's heart ached from happiness. She'd never felt this way with her ex-boyfriend. This was something different. Something deeper. Even though they had only known each other a few days, Nick knew her better than anyone else, and loved her even more because of it.

Nick wrapped his arms around her, pulling her body to his. When his lips touched hers she reached for his neck, grasping frantically at his wavy hair. Nick pressed her against the wall, kissing her cheek, her jaw, her neck. She'd do whatever he wanted her to, stay or go. She'd never believed in fate, thought it was something for the weak-minded to find comfort in. But in that moment, she knew that fate did exist. Because Nick-this Nick-was her fate.

Just then the house shook violently. Nick broke away, bracing himself against the wall with one arm, the other still laced behind Josie's back. It felt like an earthquake, or at least how Josie imagined an earthquake would feel. Only it didn't weaken. After a few seconds, the shaking was getting stronger, so violent Josie had a hard time standing upright.

Jo flung the bedroom door open. "Something's wrong."

Josie and Nick ran into the room. The portal had opened and on the other side, Josie could see Penelope and her dad standing in front of the large X-FEL rig in their basement.

Dr. Byrne had already come through the portal. She stood in the middle of Jo's bedroom, a look of panic on her face.

"I didn't know," she said to Josie's mom. "I thought my husband had Jo. I thought he'd be here when the portal opened. I . . ." She looked up at Josie. "I didn't know what to do."

The rumbling intensified. Inside the mirror, a massive hole was forming. The thick substance of the portal swirled around it, picking up speed as the hole grew larger.

Josie grabbed Dr. Byrne by the arm. "What's happening?"

"I calibrated the X-FEL to hold the portal open," Dr. Byrne said. "Until I could figure out what happened to my daughter."

"To hold it open?"

Dr. Byrne nodded. "Only it's growing too quickly. The gravitational field is accelerating beyond the bounds of its force."

Josie stared at the widening hole in the portal. "At this rate, the field will collapse in on itself."

"Isn't that what we want?" Nick said. "To close the portal?"

Josie shook her head. "This will close the portal, all right. And destroy both of our worlds in the process."

Josie's mom staggered to her feet. "Oh dear God."

"Can we stop it?" Josie said. "Shut the laser down?"

Dr. Byrne shook her head. "It's too late. Turning off the laser won't stop the process."

"Wave interference," Josie's mom said. "A wave-interference pattern will cancel out the beam."

Dr. Byrne's face lit up. "If it's of equal strength, it should disrupt the field completely, breaking the portal for good."

"Equal strength?" Nick said. "I don't suppose you've got another X-FEL lying around the house?"

Josie shook her head and smiled, remembering her science experiment in AP Physics. "But we've got some mirrors."

4:10 P.M.

Josie's mom passed through the portal first to explain to her husband and Penelope what they needed. Penelope dashed upstairs and Josie watched with a fluttering heart as her dad lifted her mom into his arms and hugged her.

On her side, Jo and Nick had retrieved the large mirror from above the fireplace downstairs and lugged it into Jo's bedroom. Once Penelope was on the other side with a similar mirror, Josie began directing their placement.

It had to be exact, which was easy when you had a variety of instruments with which to measure distance, angles, and other environmental variables. Not so easy when you were eyeballing it in a room that was literally being shaken apart.

Dr. Byrne and Nick struggled to keep the angle steady, reflecting the beam of the laser directly back onto itself. Penelope stood behind the X-FEL, her mirror catching the split photons as they passed through the portal. Together, it should be enough to create a powerful interference pattern that would disrupt the portal.

If only they could keep the mirrors from moving.

"Higher," Josie yelled. The rumbling was so intense she could barely hear her own voice. "Ten degrees."

Jo's room was literally falling apart around them. Chunks of plaster loosened by the violent shaking crashed to the floor. The bookcase pitched forward, falling face-first onto the bed like a diver doing a belly flop. The window rattled so fiercely she thought it might explode.

"It's not working," Jo screamed.

Josie didn't lose focus. She stood right next to the mirror. They were so close. "Just an inch more."

Suddenly the shaking stopped. The mirror had hit its target and Josie watched with rapt attention as the hole in the middle of the portal fluctuated, then rippled as the particle waves began bouncing against one another. Dr. Byrne and Nick held the mirror firmly, the angle perfect.

In an instant, the hole in the portal was gone. The surface blurred and the image of Josie's parents distorted. The portal was closing. This time for good.

"Let's go," Nick said, grabbing her hand.

"Are you sure?"

Nick smiled. "Hurry up."

Josie plunged into the portal, her hand clasped tightly in Nick's. Only this time the substance around her felt heavier. Less like pudding and more like setting concrete. She could barely move. Her legs strained against the portal but she wasn't getting anywhere. Her lungs ached as she began to run out of air, and she grasped Nick's hand tighter. She reached out blindly with her other hand, desperate for the clean air of her basement, but felt only the thickening goo around her.

She opened her mouth to scream but the viscous substance filled her mouth. The light began to dim as she choked and gagged.

Suddenly Nick's hand was ripped from hers; then she felt a violent shove and her body lurched toward the other side of the portal.

A hand was on her arm. Then another. Someone was pulling her into the light. With a final heave, Josie flew out of the portal onto the basement floor.

"Where's Nick?" she gasped.

Josie's dad knelt down by her side. "Jo Jo, he didn't make it."

Josie's body went numb. "What?"

"He pushed you through," her mom said. "He let go and shoved you toward us."

Her dad stroked her hair. "Then Penelope and I grabbed you and pulled you free."

Nick was gone. He'd died trying to come with her, swallowed up by the suffocating substance of the portal. She'd lost him again, this time for good.

Josie clasped the necklace in her hands, and cried.

FIFTY-FIVE.

7:10 A.M.

JOSIE'S DAD TURNED AROUND FROM THE DRIVER'S seat with a look of concern on his face. "Are you sure you want to do this?"

Josie glanced out the car window. The brick facade of Bowie Prep loomed above her, familiar and yet different. It was her school in her universe, and yet nothing about it was the same as it had been a week ago. Josie's hand strayed to the necklace that rested against her chest. Or more precisely, she wasn't the same. After all that she'd been through, the idea of facing Madison and Nick and a school full of people who still gossiped about her in the halls as she walked past suddenly seemed unimportant.

"Josie?" Her mom leaned around the front passenger seat. "You don't have to go. We don't want to rush you into this."

Out of the corner of her eye, Josie saw her dad reach out, take her mom's hand, and squeeze it affectionately. Despite her own sorrow, she was so relieved her parents had found each other again.

Josie stared out the window. She couldn't just sit at home. Every time she stopped doing something, every time she closed her eyes, she thought of Nick, and the sorrow of losing him washed over her afresh. No, she had to go back to school. It was the only way she could keep going.

"I'm ready." She opened the door and slid out of the car. "Penelope will be there. I'll be fine. I promise."

Bowie Prep felt strange as Josie slowly made her way upstairs to her second-floor locker. She remembered her first day at school in Jo's world, the excitement and anticipation she felt, combing the hallways for a glimpse of Nick. She'd expected to find a boyfriend with whom she could make amends for the mistakes in her own relationship, but instead she'd found something else.

Love.

She was no longer afraid to face Madison and Nick in the hallway at school. She was no longer afraid of the whispers and rumors. None of it mattered. Nick was gone, she would never see him again, and her love would live with her forever, a dull ache that would never go away.

She rounded a corner, lost in her own painful memories, and ran straight into her ex-boyfriend.

"Josie!" Nick said, a look of relief on his face. "I've been looking for you."

She thought she'd prepared herself for this, to see her ex-boyfriend who was the identical twin of the love she had lost, but her heart clenched at the sight of him and it took every ounce of her strength and self-control to keep the tears at bay.

"Hey," she said. It was all she could manage.

"Hey?" he said with a playful grin. "That's all you've got for me?" Nick leaned down, his lips reaching for hers.