When Kristy Thomas' mother decides to get married again, Kristy and her friends in the Baby-sitters Club have to cope with all the small children that her family and friends are bringing to the wedding.

"Something fishy is going on! The Pyrockians, those fiery little invaders from outer space, are back and they're planning something seriously strange. And what exactly are they doing with an undersea volcano? Would you believe they're evolving into diamonds? It is up to the little merman, young prince Gillbert, to figure out what those pesky Pyrockians are really up to -- but he's gone, with his best bud, Sherbert, to a strange mysterious place in...