Tilly is seven and a half. She lives with her dad, a scientist who used to invent things for the government. They are very happy, but they are both missing Mummy, who died about a year ago. So when Tilly's dad creates a time machine in their back garden, there's only one place that Tilly really wants to go: back to her sixth birthday party, when she ate too many cupcakes and her mummy was there. There's only one problem. Tilly's dad has got stuck in the past, and she must follow him through history, from battles on boats to Buckingham Palace, to get him back.