Time's Up - Mathilda Holiday, #5

Special agents, a secret bunker, and ancient magic that nobody was prepared for.. Recovering from injuries, Mattie is chomping at the bit to go and rescue her sister, Tillie, and her friend, Trevor. Especially after a new ally reveals that they're most likely being held in the same super secret underground headquarters where the leader of the entire organization is holing up.  Meanwhile, inside the bunker, Tillie and Trevor are trapped in cells with no doors or windows, and their mage powers aren't working.
Escape seems impossible, but then again .