Aldrea, a young member of the outpost the Andalite race has placed on the planet of the Hork-Bajir, must help her native friend Dak when the ruthless, parasitic Yeerks try to enslave his people.

"It begins with the dreams. Cassie has been having strange nightmares of a voice calling to her from deep beneath the ocean. They're unsettling, but it isn't until Cassie discovers Tobias has been having dreams too--the exact same dreams, of a voice pleading for help--that she decides it's time to start listening. When pieces of debris wash up on shore, marked with alien writing, the team realizes they may still have allies in the fight for Earth--allies...