Maisie Hitchins longs to be a detective. She's sure there are lots of mysteries to solve on the streets of Victorian London - if only she weren't so busy running errands for her grandmother... When a star-of-the-stage Sarah Massey comes to visit Maisie senses a mystery. Sarah is distraught - her fiance has given her a priceless emerald necklace and now it's gone missing. Maisie sets out to investigate but nothing is what it seems in the theatrical world of make-believe...