Denny Auramen, a young boy on the autism spectrum, moves in with his older sister Jenna following their parent's divorce. Finding it difficult to shoulder so much responsibility, Jenna pushes Denny to solve an ancient theorem that he believes is dangerous, in order to get him into a prestigious math program. The theorem sends the children to the world of Gossamyr, a place where math is magic and an ancient evil has been unleashed. Here, Denny's gifts make him a powerful Arithmancer, but this only complicates the stranded siblings' already strained relationship. Together, Jenna and Denny must find a way to navigate this treacherous land to not only find their way home, but back to each other.