Book summary from publisher:

Sal and Gabi have to try to make everything right with our world when there is a rogue Gabi from another universe running loose.

Sal Vidon creates wormholes. But Sal's father, a calamity physicist, is trying to shut down all the wormholes Sal creates, because Papi thinks they are eroding the very fabric of our world. All of Papi's efforts are in vain, however, because a Gabi from another universe has gone rogue and is popping up all over the place, seeking revenge for the fact that her world has been destroyed. While Sal and Gabi work together to keep both Papi and Rogue Gabi under control, they also have to solve the mystery of Yasmany, who has gone missing from school. Could it have something to do with the wormhole in the back of his locker?