Gabriella, Darren, Fiona, and Mack are Changers, a magical line of shapeshifters that can transform into mythological creatures, from werewolves and selkies to lightning birds and spirit foxes. And they've just saved the world from a warlock bent on destroying life as we know it.

Now, the Changers are training hard to control their growing powers. Something that seems easy for everyone except Gabriella. Gabriella loves the speed and strength that comes with being a jaguar, so much so that her transformations are out of control. If she changes during a soccer game, everyone at school could learn her secret.

But then Gabriella finds a green mask and an old newspaper clipping in the attic. The article is about The Emerald Wildcat, a Changer just like her who fought crime as a masked heroine. Holding the emerald mask in her hand, Gabriella wonders, does the mask belong to her mom? Could her mom really be a superhero? And more importantly, could she help Gabriella get a handle on her powers?