Ever since the death of their parents in a boating accident on the Kittiwake River when Tilly and Mole were babies, the girls have lived with their aunt Hy in a small and quiet lakeside town. Their aunt's reluctance to discuss the tragedy in anything more than the most cryptic, fragmented terms has only served to feed the sisters' curiosity -- giving rise to secret fantasies and unifying Mole and Tilly in their devoted quest for buried truths about the history that has been denied them both. But this warm and gentle summer is different from most. In this season of exploration, a subtle change is taking place that draws Mole's close confidant and inseparable companion farther and farther away from her. And others have arrived at Pillow Lake -- strangers invading a protected domain -- disturbing the delicate tripartite balance Mole, Tilly and Hy have maintained with the past for years, bringing doubt and confusion to two children on the precarious brink of adulthood while, at the same time, offering the luminous promise of understanding.