Tobias has stayed in his morph too long and must now live out the rest of his life as a hawk, but that does not keep him from contacting his friends and continuing the fight against the Yeerks, a race of hostile aliens that are invading and controlling the brains of humans.
Cassie, one of a group of kids who have been given the power to transform into animals in order to fight an invasion of the alien Yeerks, leads her friends into yet another mission when she discovers the Yeerks are trying to access Zone 91, the most secret place on Earth.
The Animorphs, a group of young people who have been given the power to change into animals in order to fight an invasion by the alien Yeerks, discover a new weapon against the aliens, but it means another trip to the Yeerk pool.
Cassie, one of a group of young people who has been given the power to transform into an animal in order to help fight an invasion of the alien Yeerks, decides she is tired of the battle, but she realizes she cannot quit just yet when a human-Controller named Karen learns her secret.
With his sister Tash and his Uncle Hoole, Zak lands on the planet Necropolis, where he hears constant talk of graves, witches, and cemeteries and finds evidence that the dead are coming back to life.
The reader's decisions control a series of adventures inside a museum, including time travel to ancient Egypt and an encounter with aliens from the Alpha Centauri system.
