As usual, I stole the summary from Cinnamon Bunzuh.
Spoiler:
Ax is sick. His Tria gland is going to explode, and if Cassie doesn't take it out, he will die. Where is Ax's Tria gland? In his brain. Hooray.
Also, Aftran (remember her?) has been arrested for treason and Visser Three is going to interrogate her and possibly find out all about the Animorphs. Keep in mind that Cassie once put Aftran inside her own head, so she knows basically everything ever.
The Animorphs try to rescue Aftran, but they don't even make it to the Yeerk Pool before Jake gets sick. Ax's illness is somehow causing everyone else to get the flu. Except Cassie, because she is the narrator. So Cassie has to do the rescue alone. She does it by morphing a Yeerk from the Yeerk Peace Movement and taking his host down to the Pool. It works, and Aftran is rescued.
Now it is time for surgery. Using her parents' tools, Cassie removes the Tria gland from Ax's brain in what is easily one of the top ten most epic scenes in the entire series. Ax is saved, but Aftran is going to die of Kandrona starvation if they don't figure something out.
The Animorphs decide to give her the morphing power. From there, Aftran traps herself in whale form and goes on to live what I'm sure is a very fulfilling life in the Pacific Ocean.