Maude and Francis try to find their father. They were surprised by a strange unknown person while launching the boat. The two children, believing to have to protect the lake Caill from the streaked moulds, as their father in a scientific article suggested it, try to convince the man to carry out a simple operation of cleaning the hull of his boat. But the American, speaking only English, rather than to try to understand them, kidnaps them since they discovered a cargo of new rifles intended for smuggling. The parents, anxious about the prolonged absence of their children, inform the authorities and a hunting for the man starts.