When their friend and step-father Bill needs to go undercover in the Middle East for his police work, it's the beginning of another adventure for Philip, Dinah, Jack and Lucy-Ann. A boat-trip along the River of Abencha — soon to be renamed the River of Adventure! — leaves the four children unexpectedly on their own with Tala the boatman and the loyal young servant-boy, Oola, as the river takes them in pursuit of a dangerous criminal. What are the villain and his gang seeking in the remote reaches of the river, and can the children bring him to justice at last?