Critically acclaimed author Emma Otheguy joins Newbery Honor-winning Adam Gidwitz as co-author of the newest adventure in the NYT bestselling Unicorn Rescue Society series as the kids travel to Cuba to help the legendary Madre de aguas. In Cuba, it is believed that a mysterious water serpent--the Madre de aguas--is responsible for providing and protecting the fresh water of the island. But the serpent is missing, and a drought has gripped the island. Uchenna, Elliot, and Professor Fauna fly to Cuba and endeavor to rescue the Madre de aguas. Unfortunately, it tries to kill them. Meanwhile, the Schmoke Brothers' goons are driving around Havana, dumping pink sludge into the sewers. What is going on? Can Elliot and Uchenna end the drought? Stop the Schmokes? Or will the creature they are trying to save just eat them instead?