Teresa reappeared as if by magic and scurried to Mr. Byrne's side. She loaded his plate with scrambled egg whites and toast, then poured him a cup of coffee and whooshed out again without a word.

Josie smiled, but her eyes drifted toward the door, hoping someone else was going to make an entrance. Where was Jo's mom?

"Is everything okay?" Mr. Byrne asked.

She hesitated. "I was just . . ." Crap, what should she say? Jo must have left something out. Maybe her mom was traveling for work? She was supposed to know that.

"I know," he said softly. "I miss her too."

Right. Jo's mom wasn't there. Phew. Would it have been so hard to mention that, Jo?

With renewed confidence, Josie helped herself to a piece of toast. Her stomach was in knots and she prayed the sustenance would calm her nerves.

Mr. Byrne ate in silence, fingering his tablet on the table next to his plate. Josie didn't say a word. This seemed to be their normal morning routine, and she didn't want to do anything out of the ordinary.

"Are you going to see Nick after school today?"

"I think so," Josie said.

"Good. I like that boy, despite what happened."

What happened?

"And it was nice of him to return the necklace to you."

Return the necklace?

Mr. Byrne shoved the last morsel of toast into his mouth and drained his coffee. "Remind him about my offer, will you? Could really use a boy like that around the office. A government job right out of high school is an excellent opportunity, even if he has to work a few years at the Grid before he could transition into my department."

Josie's head was spinning as Mr. Byrne stood up and walked around to her side of the table. Something happened? What was the Grid? And Nick returned the necklace? Josie thought it had been a gift from boyfriend to girlfriend, just like the one Nick had given Madison. What was Jo's dad talking about?

"Have a fantastic day, princess." Mr. Byrne planted a kiss on the top of her head. "I'll be home late, so don't wait up."

TWENTY-ONE.

7:15 A.M.

JOSIE'S MIND WAS A BLUR BY THE TIME SHE CLUMSILY grouped together what she assumed were her school things and closed the door to Jo's room behind her. She was missing a piece of the puzzle. Had Jo not told her something? Or was she blowing Mr. Byrne's comments completely out of proportion?

Stop trying to figure it out, Josie told herself. She was just a visitor and would be back in her own miserable life before she knew it. So what if Jo's mom was away on business? At least Josie got to spend some family time with Mr. Byrne, who was sweet and attentive and not a heartbroken mess dating a former prom queen half his age. Besides, the most important part of her day was still ahead: Nick.

Josie slipped Jo's cheat sheet out of her satchel as she headed downstairs. "Car's in the garage off the laundry room behind the kitchen." Got it. She took a right into the kitchen and tried to look easy and casual as she passed Teresa. The housekeeper stood at the sink meticulously washing the breakfast dishes. She didn't look up as Josie walked by.

As in every other part of the house, bright lights flooded the Byrnes' garage, glistening off the chrome fixtures. But Josie barely noticed the flood of lights. Her eyes were locked on the sleek black car before her.

The two-door BMW coupe looked as if it had just come off the showroom floor. Gleaming black with enormous performance wheels and a smoky glass moonroof, it looked like something James Bond would drive. She climbed into the leather driver's seat and sat there, afraid to touch anything. While her old Focus had exactly two buttons on the dashboard-air-conditioning and hazard lights-Jo's car was like the command center on a nuclear submarine. With a tentative finger, she engaged the push-button ignition. Instantly, the car came to life as the mirrors, steering wheel, and seat all moved into the perfect position for Josie's frame. The in-dash navigation screen welcomed "Josephine" with a personal greeting, and a series of flashing lights and beeps told her that everything had been checked and rechecked, and they were ready for launch. Er, for her drive to school. Same thing.

Josie almost felt sorry for Jo, who'd have to limp through suburban Bowie in the shuddering, temperamental disaster that was the Teal Monster. She probably should have warned Jo about that tricky ignition.

Oops.

It took her a few tries to find the visor button that opened the garage door; then with a deep breath and the school's address programmed into her GPS, Josie eased the car down the driveway.

"Turn left ahead," the car's mellifluous computer voice told her at the end of the driveway. Just like home, Josie turned left on Round Tree Lane. The houses looked similar on the treelined street. But also different, like everything else about Jo's world. There were a lot of brand-spanking-new McMansions, as well as totally remodeled older houses like the one Jo lived in. Originally a small craftsman like Josie's house, one here had been transformed into a modern two-story home complete with attached three-car garage.

Then there were the massive streetlights in front of every house in the neighborhood. Every single house. In addition, most had floodlights mounted on their exteriors-above the front door and the garage. Apparently, everyone was paranoid about not having enough light.

Despite the fact that this neighborhood was significantly more upscale than the one in which Josie lived, there were several houses that looked completely abandoned. Windows boarded up, lawns overgrown and gone to seed. A few even had collapsed roofs. And each abandoned house had a large sign staked into the lawn: NO GRID ACCESS.

Josie wondered what that was all about.

While Jo's neighborhood may have seemed strange, Bowie Prep, on the other hand, was relatively familiar. Same imposing brick facade, same smattering of kids scurrying to and fro. The parking lot was packed by the time she got there, except for one spot right near the front. It didn't seem to be handicapped or reserved. Someone must have just left. Finally, something was going Josie's way.

For the first time since she walked through the mirror, Josie felt at home. Bowie Prep in Jo's world looked a whole hell of a lot like Bowie Prep in Josie's, except cleaner and brighter. Still, things were in the same places-the entry hall, school office, courtyard all right where Josie would have expected them. Even her locker, number 441, was in the same place as Josie's number 441. Everything was the same.

Except one thing. Here, people noticed her.

"Hi, Jo!" a couple of girls said in unison as they passed her in the hallway. They looked vaguely familiar, though certainly no one Josie was friendly with back home, but she smiled in return like they were best friends. A group of guys greeted her around the next corner, then a trio of nervous underclassmen smiled at her tentatively as she stopped at her locker. It was like she was school royalty or something. Everyone knew Jo Byrne, and apparently for all the right reasons, whereas back home, Josie was just known as the sap who got "Byrned" while her boyfriend and best friend got it on behind her back.

Josie checked her cheat sheet for Jo's locker combination. 35-12-8. Exactly the same as her own. It was amazing the things that were the same.

And the things that weren't.

Jo had English for first period and as the warning bell rang, Josie made her way upstairs for class. Though she'd been combing the halls for Nick since she arrived, she still wasn't prepared to find him standing at his locker near the top of the stairs.

She froze, staring at him. He looked exactly as Josie had seen him in her dreams. The two Nick Fiorinos could have been carbon copies of each other. Black, wavy hair left slightly long so the thick strands were able to curl up in heavy coils behind his ears. Dark brown eyes, almost black, and heavy eyelashes longer than any boy had a right to. Strong nose and chin, right out of a Roman sculpture.

Josie felt her hands trembling and prayed she hadn't broken out in a sweat. The last time Josie had spoken to her ex-boyfriend face-to-face, he'd been shirtless in front of Madison's house, pounding on her car window, an image Josie wanted to douse with mind bleach.

But this Nick wasn't her ex-boyfriend. He was her boyfriend. She'd been in love with Nick for as long as she could remember and here, for twenty-four hours, she could fix what was left broken between them.

Nick closed his locker, and turned to find Josie staring at him. His eyebrows pinched together, obviously confused. "Jo?"

Josie's heart pounded. It was as if he'd never cheated on her, never broken her heart. A lump rose in Josie's throat and without thinking, she launched herself at him, wrapping both arms tightly around his neck.

"Whoa," he said, immediately trying to pry her arms away. "Jo, are you okay?"

Josie suppressed a sob and clung to Nick more fiercely.

"Hey!" Nick whispered harshly in her ear. "What the hell are you doing?"

Josie jerked her head up as Nick wedged his hands between them, pushing her roughly away by the shoulders. His face was hard-set as he stared at her, his eyes narrow and wary. He was clearly confused and disturbed.

No more confused and disturbed than Josie. There was something in Nick's face and demeanor that wasn't right. He wasn't the loving, gentle boyfriend Josie expected. Oh no. Josie's stomach dropped. Had Jo and Nick broken up in the twenty-four hours since she and Josie had made the plan to switch places? That would have been Josie's luck, wouldn't it? To have gone through all this just to be in the exact same space with her ex-boyfriend.

Josie took a breath and calmed herself. Maybe Nick had the same problem with Jo that he'd had with Josie? Maybe he needed to talk about something and Jo hadn't been available?

"If you need to talk," she offered, "about anything, I'm here anytime you-"

In line with Josie's luck, the bell rang for class. Nick took the opportunity to bail. "I gotta go. We'll, um, talk later?"

"Yeah," Josie called after him as he disappeared into the throng. You bet your ass we will.

This day was not going according to plan.

TWENTY-TWO.

11:15 A.M.

JOSIE DASHED INTO THE SECOND-PERIOD PHYSICS classroom as the final bell rang. Barely on time, as usual. She smiled to herself. You can take the girl out of the universe but you can't take the universe out of the girl.

She had no real excuse for almost being late. Even though she was dazed from her interaction with Nick, physics was in the exact same room as in her own version of Bowie Prep, but it had taken her freaking forever to get there. Every hall she walked down, it was like everyone made a point to say hello to her. No one approached her, no one walked with her to class, but people were falling over themselves to smile and be polite. Still, Josie noticed a distinct lack of genuine warmth in their salutations. In fact, Josie thought she detected a trace of fear from most of the students as they fired off a quick "Hi, Jo!" then scurried back into the anonymity of the crowd.

It was kind of like being the world's most popular leper-the focus of everyone's attention and no one's affection-and it made her incredibly uncomfortable. Was that what it was like to be popular? A kind of cold, fearful isolation?

But the moment she entered the physics classroom, all of her discomfort faded away. The room was an exact replica of her own beloved Bowie Prep science lab. The same metal tables and ancient stools. The same equipment lining the walls in floor-to-ceiling bookcases. Even the same students at the same tables, including the Kaufman twins in the front of the room, and the Swedish exchange student sitting prim and proper near the door. And at the same table-without a lab partner-was Penelope.

"Hi, Pen," she said, taking the empty seat next to her old friend. "How's it going?"

Penelope started and cast a furtive glance at Josie from behind the thick, black hair that hung half in her face. Josie saw in Penelope's eyes the same look she'd seen from some of the students who greeted her in the hallway: fear.

"Look, I already told you," Penelope said, her voice guarded and low, her eyes fixed on the lab table. "I'll make sure you pass, okay?"

"Um, okay."

"Just do what you promised."

Josie was utterly confused. "What I promised?"

Penelope's head snapped up. "The Grid. You promised to make sure we don't get cut off."

"Oh!" Josie smiled and tried to act like she had any idea what Penelope was talking about. "Of course. Already taken care of."

Penelope's body relaxed somewhat.

Mr. Baines clapped his hands and the class came to attention. Josie pulled a notebook and pen from her bag, and caught the look of confusion on Penelope's face as she stared at both.

The lecture was pretty standard, a lesson on fields: electromagnetism, strong nuclear force, weak nuclear force, and gravity. Practically remedial as far as Josie was concerned, so she didn't even bother to take notes. Hell, she could have taught that class. Penelope appeared to be of the same mind-set: Josie saw her occasionally scribble down a name or an equation, but for the most part she doodled on her page. At one point she caught Josie watching her draw a diagram of relativity, showing the grid lines of space warping around the mass of a giant object. Penelope quickly flipped to a new page in her notebook, and carefully shielded her page from Josie's view for the rest of class.

Bored and disturbed by the fact that Penelope was not only not Jo's friend but appeared to be terrified of her, Josie zoned out for most of the lecture, until one of Mr. Baines's questions grabbed her attention.

"Which brings us to the last question before the bell," he said, with a dramatic flourish at the equations he'd spent most of the period diagramming on the whiteboard. "And your homework assignment for tonight. Despite being the weakest of the four fundamental fields, gravity is also the most important. Tell me why. At least five hundred words."

A groan rippled through the class, and Josie was surprised. Really? That was hard? She couldn't help herself; she laughed out loud.

"Miss Byrne," Mr. Baines said, his voice full of concern. "Is everything okay?"

"Yes, sorry," she said, composing herself.

Mr. Baines's glance shifted to Penelope, then back to Josie. "Is everything okay with tonight's assignment?"

Really? The teacher was asking her if the assignment was okay? Josie looked nervously around the classroom. "No, it's fine. It's just kind of easy."

She had no idea why she said it. Perhaps her combative nature with her own version of Mr. Baines was bubbling to the surface. But as soon as the words flew out of her mouth, she regretted them. The color drained from Mr. Baines's face, and a chorus of whispers raced through the room. What had she said? What mistake had she made now?

"I just meant," she said nervously, trying to make it better, "the idea that gravity is the only field that works on all particles regardless of mass is pretty basic, right? And the only field that can unite quantum mechanics and general relativity in one . . ."

Josie's voice trailed off. Every single person in the room was staring at her, eyes wide, in several cases mouths agape. No one said a word; no one moved. They just stared at her like she was sitting there in class completely naked.

She turned to Penelope, who stared at her too, but with something more than the shock reflected in every other face. Penelope's narrow eyes were closed to thin slits, wary and alert, and her brows were scrunched low.

Josie swallowed hard. Did Penelope suspect that Josie was an impostor?

The bell rang, saving Josie from any further muck-ups. Mr. Baines repeated the assignment and scurried from the room without even a look in Josie's direction, and the rest of the students followed suit. It was as if the room were on fire, people were so anxious to get out of there.

Josie packed up her things slowly. Holy crap, she'd better keep her mouth closed for the rest of the day before she ruined the whole masquerade. She stood up and saw Penelope standing silently by the door of the empty classroom.

Her chin was jutted out and her arms folded across her chest. "What was all that about?"

Damn. Josie must have made quite a faux pas. "Sorry," Josie said. "I didn't realize I was being rude."

"Rude?" Penelope laughed drily. "You're Jo Byrne. You own this school. Hell, you have your own parking spot reserved every day. No one would ever think you were rude."

"Oh." Josie was confused. "I just thought . . . I mean, everyone got quiet."

Penelope tilted her head to the side, scrutinizing Josie. "You know why."

Josie's mind raced. "Um . . ."

"You've never answered a question in class."

So science wasn't Jo's strong suit. Of all the things they had in common, Josie found it ironic that an aptitude for science wasn't one of them. "Oh, well, you know I'm not good at science."