Jake, Tobias, Cassie, Rachel, and Marco are normal kids walking home one night. They decide to take a shortcut through the abandoned construction site, when a spaceship comes out of the sky and lands right in front of them. It opens and a wounded alien prince named Elfangor (he's an Andalite) comes out. He tells them that a bunch of slug-like aliens are already on Earth, called the Yeerks. They go into people's heads and turn them into controllers and they will continue to do so until they are stopped. The Andalite gives them a special power to help them stop the Yeerks, the ability to morph into any animal they touch, but explains they must be very careful not to stay morphed for more than two hours at a time or they will stay morphed. A Yeerk ship shows up and Visser Three, a Yeerk who has the ability to control Andalites, comes out. He morphs into a scary creature and eats Elfangor right in front of them. They run away and manage to escape and try out their morphing abilities. Tobias wants to help stop the Yeerks, but the others are afraid. But they have an advantage - every three days the Yeerks have to go to their feeding ground, so the kids know where they will be. When the time comes to actually fight the Yeerks, they all join forces and fight together. But Tobias stays a hawk for longer than two hours, and now he cannot morph back - he's become a nothlit.

The Animorphs think that Rachel's friend Melissa Chapman, who is the daughter of the vice principal, is being controlled by the Yeerks. She has been acting differently lately and not doing any of the things she normally does. In order to find out if it is true or not, Rachel decided to morph into Melissa's cat Fluffer McKitty in order to get inside Melissa's house. To acquire the DNA from Fluffer, she first has to morph into a shrew to get him out of a tree and the rest of the Animorphs have to catch Fluffer. They finally do and Rachel walks though the cat door in morph. Mr. Chapman has learned about Melissa and the Yeerks, and tries to make a deal with Visser Three. He wants him to spare Melissa and in return he will become a controller. Visser Three is meeting with Mr. Chapman and sees Rachel and orders her killed. She gets away, but returns later with Jake, who is morphed as a flea. The two of them have to try and escape, but they get caught and taken to to a coonstruction site so Visser Three can deal with her himself. Rachel needs to come out of morph soon or she will be stuck. Can Rachel get away from Visser Three in time to unmorph? Will she be stuck as Fluffer? Read The Visitor to find out!

When Tobias, Jake, Rachel, Marco and Cassie were first given the power to morph, they were told one very important thing about it: Never, ever stay in a morph for more than two hours. Tobias has been in his morph for longer than that and now he's a red-tailed hawk forever. What's worse is that he's starting to forget what it was like to be human. Inside it is a struggle between the boy's brain and the hawk's brain for control of the hawk body, and the instincts of the hawk are making it hard for the boy to win. Tobias had seen an unusual ripple in the sky one day while he was flying, but decided it was not important. Then he sees it again, only this time it runs down an entire flock of geese that were in its path. He tells the others and they realize it is a Yeerk truck ship that supplies water and air to the mother ship. They come up with a plan to bring the truck ship down by morphing into wolves, then going to the forest to acquire some trout DNA. This will let them get into the ship with the water. Meanwhile, the hawk side of Tobias gets stronger and stronger, and he leaves, letting the others plan to stop the Yeerks. Can Tobias ever gain control again? Will his hawk instincts take over completely? Can the rest of the Animorphs stop the Yeerk truck ship? Read The Encounter to find out!

It is three in the morning and Cassie has just gotten into squirrel morph when a fox tries to catch her and make her his dinner. Tobias, who is still stuck in hawk morph, swoops down to her rescue. Cassie explains to Tobias that she is awake so late because she is having weird dreams. Tobias tells her is is having some too, where a voice is calling him from the sea. The two of them realize they are having the same dream. Rachel, Marco, and Jake join them later in Rachel's room and they explain about the dream. Everyone wonders how the two of them could be having the exact same dream. They happen to see a news show from a beach, where a man has a piece of metal that has washed up on the beach. That night Tobias and Cassie have the dream again, only this time they can hear what the voice is saying to them. It says, "If you can hear me . . . come. If you can hear me . . . come." They acquire dolphin DNA and morph into them, heading out into the ocean. The Animorphs believe the voice belongs to an Andalite trying to get help and they are trying to get to it before the Yeerks do. What they do find is a huge underwater dome with trees and rivers and all the above-ground things growing in it. They morph back into humans and swim into the dome, but are fired upon by someone and knocked out. Cassie wakes up to find an Andalite pointing his weapon at her. They find out his name is Aximili Esgarrouth Isthill, Ax for short, and he is the brother of the Andalite who originally gave them their morphing powers. They leave the dome, which is really an Andalite ship, with Ax and get attacked by Vissor 3 and the Taxxons. A friendly whale helps them out and they make it to shore. They help Ax act like a human so he will not stand out. Can Ax become one of us? Or will Vissor 3 find him? Can the Animorphs stop the Yeerks from taking over everyone? Read The Message to find out!

Ax wants to go home and needs a Yeerk ship to do so. He needs the Animorphs' help to get a ship and leave. The Animorphs need to get all the parts to build a gadget to call a Yeerk ship, so they go to the mall. Ax morphs into a human so he will not stand out and they go and get the parts. Ax finds he loves the taste of the food at the mall and starts eating everything in sight. The group ends up in a grocery store, when Ax becomes scared and unmorphs. The police have been chasing the group since the mall and now they have them cornered in the store. They have only one way to avoid being caught, so they morph into lobsters in the lobster tank. Naturally, Jake and Marco almost become dinner for someone after being bought as lobsters, but luckily manage to get away. During all this Marco, who never wanted to be an Animorph but never had a choice in the matter. His life is a bit mixed up, what with his mother dying two years ago and him looking after his dad, and all he ever wanted was a normal life, is having doubts and other feelings. The Animorphs get captured and taken to the mother ship. Marco finds out his mother is still alive and even more astonishing is that she is working with the Yeerks. Worse yet, she is an Visser One, the most important officer in the Yeerk army. How does Marco deal with this? Will he be able to fight Visser One when the time comes? Or is this the very thing that will be the Animorphs' undoing? Read The Predator to find out!

Jake didn't want to be an Animorph in the first place, but he had no choice in the matter. He certainly never wanted to be the leader of the group, but he had no choice in that either. And it goes without saying that he never wanted a Yeerk in his head, but it happened and once again he had no choice about it. After spying on his brother, who was a controller, he found learned about a hospital he was very interested in. Jake and the other Animorphs went to check it out and found a Yeerk regeneration pool pretending to be a hospital. Jake fell into the Yeerk pool, although he got out quickly. The Animorphs destroyed the Yeerks, all but one... While he was in the pool, a Yeerk crawled in Jake's ear, making him a controller. Suddenly he was unable control his body and could hear voices in his head. This made Jake the enemy, the very thing the Animorphs had vow to put a stop to. It also made them realize that every time they destroy a Yeerk, they also destroy the human attached to it. Can they find a way to save Jake and destroy the Yeerk in him? If not, will they be able to stop him before he does any harm? Read The Capture to find out!