From a #1 New York Times bestselling author, a research expedition to an alien planet takes a treacherous turn for married scientists in this sci-fi fantasy. As the first humans granted permission to explore Tslamaina, Etienne and Lyra Redowl should have been ecstatic. The planet's massive river valley is like no other in the known universe, with three intelligent species living along its waters—a dream expedition for the geologist–anthropologist duo. But the intolerable climate makes their research arduous, as does the growing tension between them. Fortunately, the husband-and-wife team are well prepared for their adventure, with a state-of-the-art hydrofoil and the assistance of the native inhabitants. But nothing could have prepared them for the dangers they encounter as they make their way to the river's source. "One of the most consistently inventive and fertile writers of science fiction and fantasy." —The Times (London)