The Enid Blyton Adventure Series

Book 7: The River of Adventure (Part 1) (1x19)

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The family are going on a boating holiday in Turkey. Bill once again gets a phone call from Sir George at the last minute. But this time he is asked to find information on a small-time blundering criminal called Uma. At the hotel in Turkey they meet a boy called Oola, the badly treated assistant of a snake charmer.