Thrill-loving fifth grader Ellie "YaYa" Silver has been waiting all summer to visit the brand new indoor water park in town. She is ecstatic when her best friend, Megan, invites her to go-that is until her twin brother, Joel (YoYo), points out that Megan is going on Rosh Hashanah. Sure, Rosh Hashanah is a big deal, but so is Splash World! What will Ellie do?