The Yeerks have captured Aftran - the rebel Yeerk working for peace between humans and the Yeerk invaders. Aftran has been in Cassie's brain. When Visser Three interrogates her, he will find out about the Animorphs, Ax, the Chee - everything. The Animorphs have to rescue Aftran - to save themselves. Except they can't. Some kind of illness is going through the team - Ax is so sick that his only chance is an operation, and the other's can't even walk, let alone fight. Only Cassie is OK. So she's got to break into the Yeerk pool on her own, rescue Aftran, and then come home to do brain surgery...