1915: Ben Retallick is asked by a War Office friend to provide two traction engines for a secret expedition attempting to take two gunboats overland from Cape Town to Lake Tanganyika - more than 3,000 miles - to wrest control of the lake from the Germans. He sends engines with young Ruddlemoor as the driver, who meets a Portuguese East African nurse and takes her side against a group of white racist south Africans. Meanwhile Antonia St Anna is influential in having Ben released, when he is arrested on circumstantial evidence provided by a business rival and accused of being pro-German.