When Tobias and his friends were given the power to morph, they were also given an important warning: Never stay in a morph for more than two hours. But Tobias broke the time limit, and now he's trapped in the body of a hawk - forever. When he discovers an important Yeerk secret, Tobias knows he has to do everything in his power to destroy it. But to do so, he'll have to contend with a part of himself that's wrestling for control. A part that isn't human.

The Earth is being invaded, but no one knows about it. When Jake, Rachel, Tobias, Cassie, and Marco stumble upon a downed alien spaceship and its dying pilot, they're given an incredible power - they can transform into any animal they touch. With it, they become Animorphs, the unlikely champions in a secret war for the planet. And the enemies they're fighting could be anyone...even the people closest to them.

It all started with the dreams. But Cassie didn't pay much attention to them. She and her friends have been having nightmares ever since they acquired the power to morph. But when Cassie discovers that Tobias has been having dreams too - the exact same dreams - about the ocean, and a voice that's calling to them for help, she decides it's time to start listening. Now, she and the others have to figure out if the dreams are a message, or a trap.

Marco never wanted to be an Animorph. He didn't want the ability to change into any animal he touches. And he certainly didn't want to be one of a handful of kids that are the planet's only defense against a secret alien army. His friends know why he feels the way he does. He's worried about his dad - the only family Marco has left. If anything happens to him, his father will be alone. But something big is about to change for Marco. Something that could finally give him a reason to fight. 

It was bad when Jake found out his brother was one of them. A Human-Controller. A slave. But that was nothing compared to this. When the Animorphs decide to visit the Yeerks' new base, it seems simple. But then they get caught, and Jake falls - just for a moment - into the Yeerk pool. Now they're out, and his friends can't see it. To them, he's just like normal. But Jake is screaming for help. His worst fear has finally come true. He's become the enemy.

Ax is an Andalite, an alien, stranded on a strange planet he's sworn to defend, even though it's not his own. Since the Animorphs rescued him, he's fought at their side, and in that time they've come to consider Ax a friend. But deep inside, Ax knows he isn't their friend. He can't ever be. Andalites must always hold themselves apart, even from their allies. As the Animorphs' past actions start resulting in deaths of innocent people, however, Ax's loyalty is called into question. Now he must decide whether to reveal the reason for his estrangement - the shameful secret of his people.

There's something pretty weird going on in the woods behind Cassie's house, the place where Ax and Tobias call home. It seems the Yeerks have figured out one very important fact: Andalites cannot survive without a feeding ground. Visser Three knows the "Andalite bandits" don't feed where he does, so there can only be one other place. Now the Animorphs have to figure out a way to stop a bogus logging operation. Because if Visser Three finds Ax in the woods, nothing will stop him from finding the Animorphs.

When Marco runs into his old friend Erek, he doesn't think too much of it. He's got more important things to do, like helping to save the world. But then Marco finds out Erek's been hanging with some of the kids at the Sharing, and he starts to think that something just a little weird is going on. So Marco, Jake, and Ax decide to morph and check old Erek out. Just to see if he's been infested with a Yeerk. The good news is that Erek's not a human-Controller. The bad news is that Erek's not even human.

There's a new rumor in town. Someone has discovered an item that proves life on other planets exists, and they've been hiding it on a base called Zone 91, the most secret place on Earth. Cassie and the other Animorphs already know about life on other planets. Too well. Their enemies the Yeerks will try to access Zone 91, to find out if what's there will threaten their mission to conquer to the planet. So the Animorphs decide to pay Zone 91 and the Yeerks a little visit. But what they discover is not at all what they expect.

Rachel's got some pretty strange stuff happening. She can't control her morphing. One minute, she's doing homework. The next, she's morphing a full-grown crocodile, and - without returning to human form - she becomes an elephant. That's when the floor gives way and Rachel finds herself looking up at what used to be the kitchen ceiling. What's going on? No one's sure, but Rachel and the other Animorphs have to figure it out - quickly. Because if someone sees Rachel's out-of-control morphing, the other Animorphs are in for some serious trouble.

Almost nothing could be as bad as finding out your mother is Visser One, the most powerful leader of the Yeerk invasion of Earth. But it happened to Marco. And even though he's been handling it pretty well, he knew there'd come a time when he'd have to face her again, knowing that the Yeerk in her brain has taken his mother away. So when Marco and the other Animorphs discover that Visser One is overseeing a secret underwater project, they know they have to check it out. But Marco's not sure if this is a battle he'll be able to fight.

Jake's made a pretty amazing discovery. It seems like there may be other people who know about the alien invasion of Earth. Others who know about the Yeerks. Jake finds a website devoted to revealing the existence of the Yeerks, but he and the other Animorphs suspect it might be too good to be true. If they try to investigate the site and it's a trap, the Yeerks could find out who they are. If they don't check it out, they'll never know if they're the only ones fighting.

Ax and the Animorphs are about to have a huge problem. It starts when they decide to morph into mosquitoes in order to slip by some unsuspecting Yeerks, and ends with them stuck in Zero-space with no idea how they got there, no way back to Earth...and no oxygen. Luckily, an Andalite scout ship finds them before it's too late. But now Ax is finally with his own people. And he doesn't know if he ever wants to go back to Earth.

Tobias has gotten used to his new life. He's a red-tailed hawk with the mind of a kid. It was difficult when he first became trapped in his morph, but he's started to come to terms with things. After all, how many kids actually get the chance to fly? But now Tobias is about to make a very difficult choice: a choice that the other Animorphs know nothing about. And it could mean the difference between living the rest of his life as a hawk...and being human.

The blue box Elfangor used to create the Animorphs has been found by a kid named David. David has no idea what he has - or what it can do. But Marco does. And when he sees David with it, he knows the Animorphs have to get that box. At any cost. But what should have been a simple plan ends up going horribly wrong, leaving David is on the run with the Animorphs. Now they have only two choices: Turn David over to the Yeerks, or make him an Animorph.