Ephraim Appledore-Smith is an ordinary boy, and up until his father's stroke he lived an ordinary life. But all that changes when his family moves to the Water Castle-their ancestral home in the small town of Crystal Springs. Mallory Green's family has always been the caretakers of the Water Castle-and the guardians of its secrets. She has been raised to protect the legendary Fountain of Youth, hidden on the estate grounds. But ever since her mom left, she's stopped believing in magic. Will Wylie's family has been at war with the Applegates for generations, all because of the Water Castle's powerful secrets. But Will has rejected legend and magic, putting all his faith in science. When Ephraim learns of the Fountain, he's sure finding it can cure his dad. With Mallory and Will's help, the trio embarks on a mission that brings them deep into their families' shared history, through every secret room in the Water Castle, and on a quest that will blur the lines of magic and science, creativity and discovery, leaving readers left to wonder: Do you believe in the unbelievable?