Twelve-year-old Theo uses arcane knowledge and her own special talent when she encounters two secret societies, one sworn to protect the world from ancient Egyptian magic and one planning to harness it to bring chaos to the world, both of which want a valuable artifact stolen from the London museum for which her parents work.

Eleven-year-old Theodosia's ability to detect black magic raises her suspicions about a magician known as the Great Awi Bubu, while Henry, home for the spring holidays, discovers an artifact at the Museum of Legends and Antiquities that is coveted by every black-cloaked occultist in London.