'What a lot of islands the river makes,' said Ida. 'We must go exploring and sail around them all.' And so begins a wonderful, magical summer. Ida, Oscar and Ping are staying with Ida's great-aunt at the ancient, river-encircled house of Green Knowe. They set out to chart the river in the canoe, and soon discover that it has some surprising and mysterious secrets. A further adventure in the well-loved 'Green Knowe' series, from Carnegie Medal winner Lucy Boston.