Twelve-year-old Mizzy dreams of being a detective and she won't let anything stand in her way — not her reputation for tall tales, or her embarrassing fear of escalators, and definitely not her Down syndrome. Dumped for the summer with cousins who've outgrown her, Mizzy feels even more sidelined than usual. But when she discovers Great Aunt Jane's diaries in a locked wardrobe in their spare room, and realises her aunt didn't die in her sleep but under suspicious circumstances, everything changes. What's more … someone in the family appears to have been involved.

Under the guise of a family-tree project for school, and armed with a brand new set of Pipsqueak markers, Mizzy grabs the chance to prove herself to her cousins — and the world.