The corridor is littered with debris from the gas explosions. I have to maneuver my wheelchair around fallen pipes and ceiling panels. I'm lucky, though, that Dad's office is on the ground floor and fairly close to the lobby. I see the lobby's glass doors, just fifty feet ahead, and my heart starts thumping. We're going to make it!

But then I look up and glimpse something moving. A surveillance camera on the ceiling is turning its lens toward me, tracking my progress as I cruise down the corridor. I think of my VR program and how the virtual Brittany observed me through the camera in Dad's office. She called herself Sigma. And she said she would kill me.

Then there's a third explosion, in an office on the left side of the corridor. The blast knocks down my wheelchair, and everything goes black.

My face is cold. Without opening my eyes, I bend my right arm, trying to raise my good hand. I touch my chin, then slide my trembling fingers across my cheek. The left side of my face is wet. I stretch my hand a little farther and feel a gash under my eye. Then the pain hits me and I let out a moan.

"Adam? Are you awake?"

It's my father's voice. All at once I realize he's carrying me. My shoulders are cradled in the crook of his right arm and my legs are draped over his left. Ordinarily it would be pretty difficult to carry a seventeen-year-old this way, but my wasted body weighs less than ninety pounds. I'm like an oversized baby resting in his arms, and I feel so comfortable there I just want to go back to sleep.

"Adam! Wake up!"

Reluctantly, I open my eyes. We're on the sloping lawn in front of the Unicorp lab, which I can see over Dad's shoulder. The building's glass doors have shattered, and thick plumes of smoke are pouring out of the windows. Dozens of people stand beside us on the lawn, all staring at the ruined lab in disbelief.

I know I haven't been unconscious for very long because Dad's still breathing fast. His face is blackened with soot, but otherwise he looks unhurt. "Can you hear me?" he shouts. "Say something!"

My chest feels crushed, empty of air. My ribs ache as I inhale. "What about...Steve?"

Dad shakes his head. I look past him and see a body sprawled on the lawn. Steve's red-and-yellow Superman shirt stands out against the grass, which is vividly green in the March sunshine.

"Adam, listen to me. You're going to be all right. As soon as the ambulance gets here, we'll take you to the hospital. But before we go, I need to ask you something." Dad bends his head closer to mine. "You remember what we were talking about before all this happened? About the hacker?"

I nod.

"You said he threatened to kill you, right? But did he say he was going to attack the lab?"

I draw another breath. "No. But he...could see me. He had access...to the lab's cameras."

Dad frowns. "Did he say he worked for Unicorp?"

"He said...his name was Sigma."

A tremor runs through Dad's body. He almost drops me.

"What's wrong?" I ask. "Do you know him?"

He turns away and stares at the lab. "It's not a hacker. It came from right here. From my own computers."

Then I hear a siren. A truck from the Yorktown Heights Fire Department comes barreling up the driveway and stops in front of the lab. As the firefighters rush into the building, two ambulances pull up behind the truck. Dad tightens his grip on me and heads for the closer ambulance.

"Hey!" he shouts at the paramedics. "My son needs oxygen!"

The next minute is a blur. The paramedics shout instructions at each other. Soon I'm lying on a gurney with an oxygen mask over my face. At some point I realize Dad isn't there anymore. Lifting my head, I look past the paramedics and see him running toward the fire truck. What's going on? What's he doing over there?

Then he grabs a fire ax from a bracket on the truck's side panel.

Holding the ax with both hands, Dad heads for the laboratory. For a moment I think he's going back into the lobby to help the firefighters, but instead of entering the charred lab he dashes to a steel cabinet attached to the side of the building. Long ago, Dad explained to me what this thing was: a junction box for the lab's fiber-optic lines. All the communications between the Unicorp lab and the rest of the world-telephone calls, emails, downloads, whatever-pass through the cables inside this box.

The cabinet's doors are secured with a padlock. Dad smashes the lock with his first swing of the ax. Then he opens the cabinet and starts slashing the cables.

No one reacts at first. The people on the lawn just gawk at my father as he severs the lab's communications lines. But after a few seconds Colonel Peterson emerges from the crowd. He edges toward the junction box, waiting until Dad has shredded every cable inside. Then Peterson says, "All right, Tom. That's enough."

Dad drops the ax. Shaking his head, he strides back to the ambulance, with Peterson following close behind. As Dad approaches my gurney, he raises his hand to his mouth. He has a devastated look on his face, guilty and horrified.

That's when I realize what Sigma is. It came from right here, Dad said. From my own computers. It's something Dad created, something that lived within the advanced circuits he built, the electronics designed to imitate the human brain. It figured out a way to jump out of those circuits and invade my VR program. Then it took control of the lab's automated systems-power, heating, ventilation, security-and tried to kill us.

The paramedics have left me alone and started treating the other injured people on the lawn. Dad bends over my gurney and checks to see if I'm all right. Then he turns around and confronts Peterson. "That was a waste of time, wasn't it?" he hisses. "I cut the lines too late?"

The colonel nods. "I'm afraid so," he says in a low voice. "Our friend has already escaped from his cage."

"He's on the Internet?"

Peterson nods again, then reaches into his pocket and pulls out his cell phone, which is apparently working now. "He sent an email to Cyber Command headquarters five minutes ago, right after the last explosion. My men are trying to trace where it came from, but it looks like the message ping-ponged all over the globe before it arrived. He could be anywhere by now."

"What did the email say?"

Peterson holds up the phone and reads from its screen. "'My name is Sigma. This message is a warning to all government leaders and military commanders. I have the power to annihilate you.'"

CHAPTER.

4.

I wake up the next morning in a hospital bed at Westchester Medical Center. I recognize the place right away-the hospital is close to Yorktown Heights, and I go there for all my checkups and treatments. Specifically, I'm in a private room in the children's hospital. The building is sleek and modern, and several of the doctors there specialize in treating muscular dystrophy.

The last thing I remember is riding in the ambulance. The paramedics must've sedated me after we left the Unicorp lab. Now an oxygen mask is strapped to my face and an IV tube hooked to my useless left arm. My chest still hurts, but not as much as before.

I feel strong enough to breathe on my own, so I reach for the mask with my good hand and take it off. Then I turn my head on the pillow and look around. Aside from the machines monitoring my vital signs, the room is empty. I'm not surprised that my mom isn't here-she hates coming to the hospital because it upsets her so much-but I thought I'd see Dad. He was in the ambulance with me, stroking my hair as the paramedics put me to sleep.

I lift my head and look for the call button to summon a nurse. Before I can find it, the door to the room opens. I expect to see my father, but instead a bald girl in a hospital gown steps inside.

The girl quickly shuts the door behind her. She's skinny and short, only five feet tall, and about the same age as me. As I look closer I notice she isn't completely bald-there's some black fuzz at the top of her head. There's also something wrong with the left side of her face. Her left eye looks swollen, almost squeezed shut, and her lips are bunched in the left corner of her mouth. I don't know what kind of illness she has, but it looks serious.

As the girl steps toward my bed, her bunched lips form a lopsided smile. "I knew it," she mutters, slurring her words a bit. "You're Adam Armstrong, aren't you?"

"What?" My throat is sore. I can barely whisper. "How do you-"

"I was a year behind you at Yorktown High." She stops a few feet from my bed. "I'm Shannon Gibbs, remember? We were in the same biology class."

I study her face, trying to place it. When I took biology in tenth grade there was a petite freshman girl who hardly talked to the other students but constantly pestered the teacher with questions. I didn't pay much attention to her because she was a year younger, but I noticed she was smart. She was the only kid in biology who got higher grades than me.

"Okay, hold on, I'm remembering something. Did you do an extra-credit report? On the nervous system?"

Her smile broadens. "Yep, that was me."

"You made those clay models, right? Of the brain and the spinal cord?"

Shannon laughs. "Oh God, those models! I was up all night making them."

"It was worth the effort. They were very realistic. Truly disgusting."

"And wouldn't you know it? That's where I got my tumor. Right where the brain connects to the spinal cord. Ironic, huh?" She taps the back of her head, just above the neck. "The cancer messed up the nerves in my face, and the chemo made my hair fall out. That explains my lovely Frankenstein look." She does a monster imitation, widening her eyes and flailing her arms. Then she points at me. "I remember your report too. Wasn't it also about the brain?"

I nod. "The brain's limbic system. Where all our emotions come from. The hippocampus, the amygdala, and the cingulate gyrus. The tangled tongue-twisters of hate and love."

"Yeah, I remember you put a ton of jokes in the report. You were funny. Definitely the funniest guy in the class."

That was my strategy back then, playing the class clown. I cracked jokes and drove my wheelchair at breakneck speed down the hallways and generally behaved like an idiot. I didn't want anyone to feel sorry for me, so I acted as if I didn't care. As if I wasn't dying. "I was trying too hard. Your report was better."

Shannon comes closer and sits down on the edge of my bed. It's kind of a forward thing to do, especially after barging into my room uninvited. She smiles again. "Don't worry, I'm not gonna put the moves on you."

I smile back at her. "That's good. I can't really start a long-term relationship right now."

"Me neither." She shakes her head. "My tumor is a pontine glioma. In plain English, that means 'Good-bye, cruel world!'"

I can't think of anything to say in response. Shannon's dying too. We're in the same boat. I'm not happy to hear it, but at least I understand her a little better. She's dying and she wants to talk. Maybe she thinks I can give her some advice.

"I saw you when the paramedics brought you in yesterday," she says. "My room is across the hall and my door was open. You were unconscious, but I caught a glimpse of you before they wheeled your gurney into your room."

Her eyes are dark brown. Above them, the wispy remnants of her eyebrows look like apostrophes. As I stare at her, I remember what she looked like in biology class a year ago: a pretty fifteen-year-old with shoulder-length black hair and dimples in her cheeks. She's still pretty now, despite her swollen eye and twisted mouth. I want to tell her this, but I'm too chicken. "It's weird," I say instead. "This is a weird coincidence, don't you think?"

"What do you mean?"

"I mean, us being here on the same floor of the hospital."

Shannon stops smiling. "It's not a coincidence. Your dad arranged it."

"Arranged what?"

"Wait a second. You seriously don't know about this?"

I shake my head. I'm bewildered.

"Your dad got in touch with my parents through the high school and told them there was a new treatment we could try. It was experimental, something his research lab had developed for you, but he said it might also be useful for other teenagers with terminal illnesses. He said he was recruiting kids to test the treatment and would explain everything to us at the hospital."

It doesn't make sense. I never heard Dad say anything about a treatment he'd developed for me. I can't even see how he'd be able to do it. He's a computer scientist, not a medical researcher. "I'm sorry, but this is the first I've heard of it."

Shannon bites her lip. "Now I'm confused. Is there a treatment or not?"

Lowering her gaze, she looks down at the bed, which is covered with a thin, white blanket. Her eyes turn glassy, and for a second I think she's going to cry. She's clearly invested a lot of hope in whatever promises Dad made to her parents. It might be a long shot, but it's all she has.

My chest aches. I don't want Shannon to lose her last hope. I furrow my brow, trying to figure out what Dad is up to. I remember the conversation we had in his office before everything went haywire, and what Colonel Peterson said about Dad's research. And something comes back to me. "You know what I think it is? It's nanotechnology. That must be what Dad has in mind."

She looks up, cocking her head. "Nanotechnology?"

"Yeah, the science of building very small things."

"I know what nanotechnology is. I did an extra-credit report on that too."

I use my right arm to roll onto my side. I feel like I need to sit up if I want Shannon to take me seriously. "Okay, my dad works with the Department of Defense, right? And yesterday he got a visit from this colonel in the U.S. Cyber Command. This guy mentioned a laboratory called the Nanotechnology Institute. He said they were doing some amazing work there."

She gives me a skeptical look. "I did a ton of research for that report, and I never heard of that lab."

"Well, this is classified government work. Very hush-hush. I'm probably breaking all kinds of laws by talking about it." I manage to prop myself up to a sitting position, but the thin blanket falls down to my hips and I notice with dismay that I'm not wearing anything underneath. I quickly tuck the blanket around my waist. "Anyway, Colonel Peterson said this institute has developed microscopic probes that can be injected into the brain. And if they've already done that, who knows what else they can do? Maybe they also have nanoprobes that can repair genes. Or kill cancer cells."

Shannon still looks doubtful. She rises to her feet and starts pacing across the room. "I read about nanoprobes for my report, and I don't think the technology is that advanced yet. Scientists can make simple things, like tiny spheres or rods or tubes, but no one knows how to make microscopic killing machines."

"Look, my dad can clear this up. I'm sure he's in the hospital somewhere. He probably went to the cafeteria to get a cup of coffee. As soon as he comes back, we'll talk to him." I try to catch Shannon's eye as she paces back and forth. "I'll tell you one thing for sure-Dad lives up to his word. If he promised you something, he'll definitely come through."

She doesn't respond at first. She keeps her head down while she paces, as if she's looking for something she dropped on the floor. Then she lets out a sigh. "All right, fine. I'll wait to hear what your dad says." Without missing a step, she points at the door to my room. "That Colonel Peterson you mentioned? Is he somewhere in the hospital too?"

"I don't know. Why do you ask?"

"When I sneaked out of my room to come here, I noticed a few soldiers in the corridor. They were standing at attention near the elevators."

This is news to me. And not good news either. Why are there soldiers at Westchester Medical Center? Is Peterson expecting another attack? Will Sigma track me down and try to kill me here?

While I worry over this, Shannon keeps pacing. I notice that she's waddling a bit, lurching to the left. It reminds me of the way I used to walk before my legs stopped working. That's another thing we have in common. "So are you still going to Yorktown?" I ask. "Or did you withdraw from school?"

She finally stops pacing and turns toward me. A bead of sweat trickles down her scalp. "My mom wanted to pull me out, but I said no. School keeps me sane. I'd go crazy if I did nothing but chemotherapy."

"But don't the drugs make you tired?"

She shrugs. "Yeah, it's hard to concentrate sometimes. But I still get the highest grades in my class."

I'm jealous. I wish I'd stood up to Dad and insisted on staying in school. I went along with him because he was so worried about my breathing problems, and because he promised to let me use his computers at work whenever I wanted. But I didn't realize how lonely it would be. Once I was out of school, no one stayed in touch. The emails and texts from my friends dwindled, then stopped. It was easier for them to forget about me. Even my best friends, the ones I'd known forever.

Shannon sits on the edge of my bed again. I swallow hard, preparing to ask her another question. I suspect the answer will be painful, but I need to hear it. "Do you know Ryan Boyd? He's on the football team."

She nods. "Sure, I know him. Big dude, good-looking. He hangs out with the other football jocks."

"How's he doing? I saw his name in the last issue of the school newspaper. He just won the Sportsmanship Award, right?"

Shannon leans closer, eyeing me carefully. "You were friends with him, weren't you? Now that I think of it, I remember seeing you talking with him by the lockers every morning before first period."

"Oh yeah, we go way back. But, you know, we haven't talked in a while."