Archie's cousin, Thistle, is about to start his apprenticeship at the Museum of Magical Miscellany. But when it comes to his initiation, the firemark that burns into Thistle's hand is a strange one, and Archie and Bramble are given it too. The Golden Circle is the mark of an ancient alchemist's club and when Archie and his cousins learn about a curse that threatens their beloved museum, they have no choice but to start their own alchemist's club, and face the darkest kind of magic.