Jack can't understand why boys get a bad rap for being gross, especially when compared to his often sticky and slime covered little sister Isabel, who he has secretly nicknamed Grossy. According to Jack, Grossy is the messiest girl on earth, and he has had it up to his nose with the mysterious odor that follows her around. Discovering the source of Grossy's smell becomes an investigation worthy of enlisting help from his friend, whiz-kid Evan. If Jack is lucky, he and Evan will uncover the source of Grossy's unknown offensive stench. If not, he may face and long and foul-scented future.