This book acts as a follow-up to 'The Departure'. Aftran, the Yeerk who was convinced by Cassie to seek out another path for the Yeerk species, has been accused of treason and will be interrogated by Visser Three within a few days. Cassie finds an unexpected Controller ally, but before the mission can go off there is a problem (of course!).
Ax comes down with a sickness that is in some way attached to morphing. He runs a high fever and is likely to die without brain surgery. The other Animorphs are susceptible to it, but, for reasons of convenience, the illness behaves like a normal flu virus.
Cassie has to carry out the impossible - rescue Altran or risk exposing themselves and the Yeerk Peace Movement, and save Ax's life with obviously no real surgical knowledge, let alone any Andalite physiology.
I'm not taking any points away, because I did like the book, but Erek and the Chee are also conveniently useless in times like these. They can project holograms, but they can't restrain an ill person from running out of their sick bed because of their programming?
This book also fares better when I think about what's coming.
Next: 'Elfangor's Secret'
Previous: 'The Experiment'