The Yeerks, a parasitic alien race bent on enslaving the Earth, have discovered a way to cause a person to demorph. With this device, they plan on capturing the Andalite bandits. The Animorphs have to get rid of the device, or be captured by the Yeerks.

The fact that his mother has been infested by an alien parasite and his father is planning to remarry puts Marco under such stress that he begins to morph into bizarre creaturees that he is afraid will attract the attention of the Yeerks.

In the beginning, Byx's original quest was to discover if there were more of her kind, or if she was destined to become an endling-the last dairne alive. She did indeed find more dairnes, and along the way she also created allies among other creatures in her world, including humans, felivets, raptidons, and wobbyks. But Byx and her new friends soon learned that it wasn't just dairnes in jeopardy of extinction, but that everyone was at risk. With the...