It is the summer of 1851 and Charlotte Morrison is on holiday in Germany with her brother and his wife. On the surface, Charlotte is an unmarried aunt with a sparse, unfulfilled life. But beneath that quiet respectability lie unsuspected depths hidden murmurings. On a day trip boating down the Rhine, Charlotte sights a fellow traveller, Edward Newman, who releases the hissing floodwaters of her subconscious. Dark and dangerous, they sweep Charlotte towards the watershed of her life, stretching her imagination to its limit; almost to breaking point.