Harry, Milly and his new friends continue their travels on the narrow boat Lady Rose. An emergency means they have to take an important cargo of bricks down the Oxford canal to Lady Margaret Hall, but on their travels they find themselves caught up in all sorts of adventures. Who has stolen the Government's secret plans and what are the two Dutchmen doing on the Oxford canal during the war? Can Harry, Susu, Lily and Finley work out what is going on?