When fourteen-year-old Jo Tiegan receives an oval mirror as a gift from the old owner of an antique shop, she is surprised to see not herself in the mirror, but another girl named Louisa. Louisa can also see Jo. It turns out the two live in the same house and share the same bedroom, just not in the same time period. When they accidentally discover they can travel to each other's time, it sets off a whole chain of events.