After Lady discovers Diamond and Pearl are not her real bodyguards, she is asked to return home. Having decided that Team Galaxy is too dangerous, instead she reteams with them to try and save all of Sinnoh Country. This splits up the team which actually markedly improves the book.
This volume focuses heavily on poor Diamond. He really gets to step out on his own, outside of Pearl's shadow, and it makes for some entertaining reading.
Still, I wouldn't recommend reading this series unless your kid asks you to.