Mystery on the Marsh is an adventure story set in the 1920s. Ned runs away from his home in the East End of London when his stepfather threatens to get rid of his beloved old dog. He and Toby make their way towards the River Thames where Ned thinks he might find work but they end up living rough on the marshes. For some months he survives by doing odd jobs for people, and working at the weekends in the café where the owner, Bessie Brown, is kind to him. But before long he becomes aware that men who speak in foreign tongues visit the marshes by boat at night and he realises a gang of smugglers is operating along that part of the coast. He makes friends with a boy who lives in a pub at the river's edge and they begin the hazardous task of tracking down the source of the smuggling, with dangerous results…