From the back cover:
"You know those machines outside of almost every grocery store ever that have plastic bubbles filled with toys, stickers, and bouncy balls? Of course you do! Well, guess what? In the World of Make-Believe, those bubbles are really real creatures, and those machines are where they live! Oh, but there's one tiny problem. A big old giant is "borrowing" the bubble friends, just like we treat the toys in the real world. Looks like Posey and I are going on another adventure to save those bubbles before they go pop!"