Three weeks after the events of Rainbow Rocks, the sirens are desperate. Without their hearts—the jewels that hold their magic—they have no power. They can't sing, can't manipulate people, and worst of all, can't feed. Their only hope is to restore their hearts, and they need to do it soon, before all three of them starve.

The problem is, they're not sure how to fix the hearts. All they've managed to figure out is that they'll need magic to do it, and magic is exactly what they don't have. Scared, starving, and completely out of options, the sirens turn to the last people who would ever want to help them, and the only ones who actually can: the Rainbooms.