Cassie's Had it. After the last mission, she realized she's getting tired of missions, tired of battles, tired of being an Animorph. She decides that she just can't do it anymore, so she quits, but that's not the worst that's happened. It seems a Human-Controller named Karen followed Cassie after the last run-in with the Yeerks and she knows Cassie is an andalite or human. Either way, if she exposes Cassie, it's all over. No more Cassie, no more Animorph, no more Planet Earth.