Traveling through time is kind of like dying.

It's terrifying. You never know when it's going to happen. When it does happen, it hurts so bad you don't want to survive.

Gabe just burned to death in a car wreck, Natalie was shot, I don't know where Justin is, and Billy was just possessed by a Xua. Usually Aerithin is here by now, but he's late, and I'm running.

The Xua are right behind me.

I've failed. Again. For the fourteenth time.

Back in the beginning, I didn't know who had attacked us or why. Aerithin tried to explain things to me, or at least as much as he thought I needed to know. The problem was I didn't always believe him, especially when he was talking about "cascading events." I thought he was talking about some weird alien religion, not a scientific anomaly.

Apparently, there are certain things that can never be changed, no matter what you do or how hard you try. Cascading events are like destiny. These are the events that set other things in motion. The launch of the Valiant, my meeting Aerithin, my supposedly meeting some guy named Noah in the future — according to Aerithin, those are all unchangeable.

I can't change them no matter how hard I try.

I know, because I didn't believe him. Not at first.

My first three jumps through time, I tried to stop the Valiant launch. I thought it was the pivotal event that needed to change in order to save our world, because the launch always led to Gabe's death. Save Gabe, save the world, right? It made sense to me, but the cascading-events thing always got in the way.

Normal people live and learn. Not me. I watch everyone I love die and I learn.

In the distance, a skyscraper tumbles to the ground, dust and debris flying into the sky, shadowing the city of Los Angeles. The sky darkens, and I can feel the end, can taste it on my tongue.

Right when I think I'm toast, Aerithin appears. He calls to me, and I jump onto his steed, that fiery lionlike beast, and together the three of us gallop away. There's only one place to go — back.

In an instant, we're racing through the Corridor of Time that separates the future from the past, and I think we're safe.

Then I glance over Aerithin's shoulder behind us and see that the army of Xua is chasing us, all of them running as fast as the beast we travel upon. Some are running faster.

All the breath leaves my chest. No. They've never done this before. They've never been able to follow into the Corridor.

And then I realize —

The Xua have learned, too.

"Faster!" I yell, leaning forward, my hands gripping the beast's long fiery fur.

It cries back with a thunderous roar.

The faster Xua are gaining ground, and I don't think we're going to make it, but we have to. If we don't, it's all over for everyone on Earth. The Xua will win, and everyone I know will be dead.

The door to my past opens up ahead of us. Just a little bit farther and I'll be there.

But the fastest Xua have already climbed onto Aerithin's fire-beast. One of them wrestles with Aerithin, trying to dislodge him, while the other Xua grabs at me. Its long fingers latch and snarl into my hair. I scream, turn, and bite its hand. Its glowing blood sprays on my face.

I'm not going to make it; I know it. They're going to kill us both.

"Jump, now!" Aerithin yells. His steed slams to a halt, and I fly off, tumbling toward the open doorway ahead of us. I roll, then hit the ground running. Another version of myself stares back at me through the mirror and, for an instant, I feel like Alice in Wonderland staring through a magical looking glass.

Behind me, Aerithin howls in pain, a horrific sound that makes me shake. I can tell by his soul-wrenching cry that he's in torment. They've caught him and they're probably killing him and I can't stop it. I have to escape. If Aerithin dies, I'm the last chance for my world —

I stretch one hand toward my reflection, and as soon as my hand touches hers, we merge. It feels like I've been slammed against a wall, like my bones are poking through my skin, and I'm being turned inside out. But I have no choice. It's this or we all die.

I'm in the past.

I'm crammed back inside my own skin.

For a few brief moments, I can still hear Aerithin screaming. Then it's quiet, except for my breathing and my heartbeat, except for the panic that surges through me.

My hands tremble.

I made it.

But they got Aerithin. Nobody yells like that unless ...

Please, don't let him be dead, I beg, even though I fear it's already too late. If he's dead, this is my last life, my last chance to save my brother, my last chance to save everyone.

Instinctively, I listen for the low growl of enemy ships circling through the skies overhead. The Xua have never come to Earth before the launch, but I don't know what to expect. None of this has happened before. They've never followed us through the Corridor of Time. They've never caught Aerithin.

Have they finally learned how to change destiny?

So many things I've gotten wrong, so many times I've failed. If this is my last chance to get it right, I'm screwed, because the Xua are already a step ahead of me.

The first time I saw one of the aliens — the Xua — I thought they were all the same. They look alike with their long arms and yellow eyes and glowing silver skin, but they're not. Not at all. There are three types of Xua, and Aerithin made sure I understood the difference, because knowing exactly which type I was dealing with in a situation meant the difference between living and dying.

The first are the Jumpers. These aliens are the foot soldiers of the Xua army. Fast and determined, they're the first to turn into a vaporous smoke and enter a human host through the person's mouth. You'll know when a human is possessed by a Jumper — their muscles tense, their jaw hangs loose, and they hunch forward when they walk, like they're on a mission.

Jumpers can't hide what they are. There'd be no point anyway, because as far as they're concerned, they will possess you.

Second are the Hunters. They're the Xua's special-ops soldiers. Hunters are highly intelligent, methodical, and nearly impossible to evade. Unlike Jumpers, when they possess a human, they're skilled enough to manipulate their host's body. You'll never know there's an alien standing in front of you until it's too late. They can track a person for miles — that's their primary purpose, and they're extremely good at it.

A Hunter possessed Billy once. That's how he was able to find Gabe and me.

I need to avoid Hunters at all costs.

Finally, there are the Leaders. All Xua answer to them. These aliens are calculating and controlled, planning every attack and leading every battle. They don't care how many Xua they lose in a skirmish, as long as they win. A Leader led that first attack on my neighborhood.

Like Hunters, you'll never know a human is possessed by a Leader unless they want you to, though in my experience, they don't seem to care whether I know. And why would they? They're just as deadly as Hunters, and they have an army of Xua at their backs.

The possession itself is horrible. Being taken over by a Xua is like being possessed by a demon: you have no control over what you say or do. No matter which type you're dealing with, if a Xua takes full possession of...