An intense, atmospheric novel about the devastating power of friendship, set against the backdrop of two cataclysmic events.

After Marissa loses her mother at six, the most intimate relationship of her life begins. Her marine biologist father, determined to channel his grief into completing his wife's research, whisks her across the globe to Thailand. There she meets Arielle, and a fairytale friendship takes hold. During the week, the girls live at the resort owned by Arielle's parents; on the weekends they join the tight-knit community of researchers on a nearby island. Together the girls discover the fragile wonders of its reefs, forests, and beaches. Together they learn to dive into the deep, holding their breath for minutes at a time, as effortlessly synchronized as the manta rays they come to know by name. Together they learn to swim their way out of danger. But then comes a wave Arielle can't outpace, leaving Marissa gutted with loss.

Years later, Marissa is back in New York, adrift and haunted by the memory of her friend. Over the course of two fateful days, as another cataclysm approaches the city and the past comes flooding back, she discovers how to sustain herself in a precarious world.