A single Arn, Quafijinivon, the last member of the race that created the Hork-Bajir has survived. He has seen his entire people wiped out. He has seen the Hork-Bajir enslaved. Now, after a lifetime of planning and plotting, he wants to get his revenge on the Yeerks. And he wants to atone for the problems the Arn have caused. He is old and weak. But he has something the Yeerks have reason to fear: the Ixcila, the mind pattern and persona of Aldrea-Iskillion-Falan, Prince Seerow's daughter and one of the main characters of The Hork-Bajir Chronicles.

Dak Hamee's Ixcila did not survive. Aldrea's Ixcila is a powerful thing for the Arn to have. Because he's also learned that while Aldrea and Dak were alive, they'd managed to steal and hide an entire transport of Yeerk weaponry. And now Quafijinivon wants the free Hork-Bajir on Earth to have the weapons to fight against the Yeerks. But he has to find the weapons first. And only Aldrea would know where they are hidden.

The free Hork-Bajir notify the Animorphs of this Arn's arrival on Earth. When they meet, Quafijinivon feels that Cassie is the best candidate for Aldrea's Ixcila. Now, Cassie has within her own mind the mind of Aldrea. She, the other kids, the Arn, and Toby, (the young Seer of the free Hork-Bajir colony), must travel to the Hork-Bajir home world, locate the weapons cache, and help launch a new Hork-Bajir liberation movement. One problem: The Aldrea persona must voluntarily surrender its hold on the host mind or decide to push the host aside. Did Aldrea's Ixcila choose Cassie because it sensed she was weak? Or was Aldrea drawn to the truly strong member of the Animorphs?