At age 13, Tina had resigned herself to being an only child. She is thrilled when her parents manage to adopt a Scottish orphan and bring her home to live with their small family in rural Ontario. Catriona is just a few months younger than Tina and seems like a perfect fit. Catriona is eager to assimilate in her new Canadian surroundings and learns to ski and swim, and quickly becomes part of a tight threesome along with Tina's best friend Sophie. Tina's universally disliked art teacher, Miss Plunk, and her amateur magician brother Irwin Plunk seem to take an unusually strong aversion to the orphan, and the feeling is reciprocated. Tina begins to sense that all is not completely normal with her adopted sister. She finds herself holding secrets from her parents as she and Sophie are introduced into a world of magic, adventure, and danger.