John Smith seems like an ordinary teenager, living a normal life with his guardian Henri in Paradise, Ohio. But for John, keeping a low profile is essential, because he is not an ordinary teenager. He’s an alien from the planet Lorien, and he’s on the run. A group of evil aliens from the planet Mogadore, who destroyed his world, are hunting anyone who escaped. Nine Loric children were sent to Earth to live in hiding until they grew up and developed their Legacies, powers that would help them fight back—and help them save us. Three of them are now dead. John is Number Four, and he knows he’s next.

In the beginning, nine aliens left their home planet Lorien when it fell under attack by the evil Mogadorians. They scattered on Earth and went into hiding. They look like ordinary teenagers but have extraordinary powers. The Mogs have come for them. They killed Number One in Malaysia, Number Two in England, and Number Three in Kenya. They tried to get John Smith, Number Four, in Ohio—but they failed. Now John has joined forces with Number Six, and they are on the run. But they are not alone. Already John and Six have inspired Marina, Number Seven, who has been hiding in Spain. She’s been following the news of what’s happening, and she’s certain this is the sign she’s been waiting for. It’s time to come together.

The stakes are higher than ever as John, Six, and Seven try desperately to find the rest of the Garde before it’s too late. The Mogadorians who destroyed the planet Lorien continue to hunt down the Garde, the small group of Loric survivors who have taken refuge on Earth. The Garde must come together. They are Lorien and Earth’s only hope.

During the dangerous mission at the Mogadorian base in West Virginia, John found and rescued Nine. But even with their combined powers, special abilities known as Legacies, the pair barely escaped with their lives—and they lost Sam in the process. In order to save our world and their own, John and Nine must join forces with Six and Seven who have been battling the Mogadorians in Spain, and who are now trying to locate Number Eight in India. Power in numbers will save us all.

John Smith—Number Four—thought that things would change once he and Nine reunited with the others. But he was wrong. After facing off with the Mogadorian ruler and almost being killed, the Garde realize they are drastically unprepared. Now they're hiding out in Nine's Chicago penthouse, trying to figure out their next move.

Even with the return of an old ally, these superpowered teens aren’t strong enough yet to take on an entire army. To defeat their enemy, the Garde must master their Legacies and learn to work together as a team. And when they receive a sign from Number Five—the last missing member of the Garde—they know they are close to all being together finally. But could it be a trap? Time is running out, and the only thing they know for certain is that they have to get to Five before the Mogs do.

The Garde have suffered an unbearable loss. Number Five has betrayed them. Eight is gone forever. Ella has been kidnapped. The others are now scattered. In Chicago, John makes the unlikeliest of allies: a Mogadorian named Adam, who has switched sides. Adam knows the Mogs secrets, and he knows where to hit them: their command base near Washington, DC. But during the assault, John learns he and the Garde might be too late. The Mogadorians have commenced their ultimate invasion plans. Meanwhile, Six, Nine, and Marina make their way through the Florida Everglades, hot on the trail of the traitorous Five. With the development of a new Legacy, Marina finally has the power to fight back—if her thirst for revenge doesn't consume her first.

The Garde may be broken and divided once more, but they will not be defeated. As long as one still stands, the battle for Earth's survival is not lost.

For years the Garde have fought the Mogadorians in secret, but now the invasion has begun. John is on the front lines in New York City. The odds may be stacked against him, but his best friend, Sam, has inexplicably developed a Legacy despite being human—and he isn’t the only one. As the pair tracks down Five and Nine amidst the chaos, they encounter another teen with powers. Whether she is a friend or a foe, however, remains to be seen.

Meanwhile, Six, Marina, and Adam are trapped in Mexico. They fought their way into the Sanctuary and were able to awaken the force hidden within, but they don’t have any way to reach the others…and the Mogs are coming for them in full force. The Garde are stretched thin. Their only shot at saving the world is to eliminate the Mogadorian leader once and for all—but his fate is now irrevocably tied with Ella’s. The Garde can’t destroy one without the other. And if they can’t find another way to stop the Mogs, humanity will suffer the same fate as the Loric: annihilation.

The Garde didn’t start this war, but they’ll do whatever it takes to end it once and for all. . . .

The Mogadorians have invaded Earth. Their warships loom over our most populated cities, and no country will risk taking them head on. The Garde are all that stand in the way, but they’ll need an army of their own to win this fight.

They’ve teamed up with the US military, but it might not be enough. The Garde need reinforcements, and they’ve found them in the most unexpected place. Teenagers from across the globe, like John Smith’s best friend, Sam, have developed abilities. So John and the others must get to them before the Mogs, because if they don’t their enemies will use these gifted teens for their own sinister plan.

But after all the Mogadorians have taken from John—his home, his family, his friends, and the person he loves most—he might not want to put any more lives in danger. He’s got nothing left to lose, and he’s just discovered he has been given an incredible new Legacy. Now he can turn himself into the ultimate weapon. So will he risk his life to save the world, or will he realize that power in numbers will save us all?