Estrid reveals that she was never in the military, but she learned to tail-fight from her famously-skilled brother. She was a young genius and was discovered by Arbat at the university after initially being ignored as a result of her gender. She took up plintconarhythmic physics under his guidance, and she has created a programmable prion virus (the vial that Ax puts down) that will destroy the Yeerks - and very possibly wipe out humans as well. Arbat finds them and traps them, and Ax explains to Estrid that their mission does not officially exist and is not sanctioned by general Andalite society like she was led to believe. Arbat takes the vial of the disease and leaves them. As Estrid apologizes, the Animorphs are revealed to be on board, as their "split-up" was simply a ploy to get Ax in with the Andalites. The Animorphs free them, and the group of seven finds evidence of Arbat and follows him to the Yeerk Pool in human form, except for Tobias, who stays as a hawk.