What lies hidden in the mud? What might happen if you look at it too long? And what will happen if you let it go?

Twelve-year-old Nina is not happy. Her scientist parents are spending a year in Antarctica. And Nina's being sent to London to stay with her Aunt Bee, an intertidal archaeologist, who lives on a converted barge on the Thames. She's also a keen mud larker, combing the river mud for fascinating, long-forgotten articles from past lives.

Nina arrives in London with an Attitude. Her parents have never left her behind before. It takes time for her to settle in, helped by the MudPuddlers, a local group of enthusiastic amateur mud larks, and especially by Molly, an elderly MudPuddler living on a nearby barge. Molly draws Nina into the magic and mystery of the ancient river and its treasures.

When she finds herself stranded in time, in the Blitz in 1940, Nina and a very unwilling fellow traveller, Tom, become runaways, fumbling their way across wartime England, desperate to return to London. Will they ever see their families again?