This book goes even further into the relationship between Tobias and Ax, and making steps towards their being family. We found out in 'The Pretender' (with a very obvious hint in the first 'Chronicles' book) that Tobias is the son of Elfangor during his time on Earth. The story hasn't had the opportunity to go into the deeper meaning of that.
The real thrust of this book was, of course, the heist surrounding the destruction of the Yeerk-developed 'anti-morphing' ray. The Animorph's plan is pretty clever, but of course involves one of them being tortured. There's a brief foray into 'Pretty White Yeerks with Problems' as well.
The real enjoyment of the story is in Tobias and Ax's bonding, and, finally, some momentum between Rachel and Tobias.
Next: 'The Prophecy'
Previous: 'The Separation'