Siren Phillips has lived her life thinking she's an ordinary girl, in an ordinary town. On the eve of her twelfth birthday, however, she learns that she is far from ordinary. Destined to turn into a Mermaid at the age of twelve, Siren must struggle with her new reality, saying good-bye to her mother and friends, while she transitions into the water. To make matters worse, a group of hunters are trying to catch her. She soon becomes friends with Adam Wilts, a new boy at school, who she turns to for help. He agrees to help her, but finds himself caught in the middle, when it's his own father who is after Siren. When Siren's mother is taken, the town must rally behind her, and help her make a peaceful transition into the water, before the hunters can find out where she is.
米尔德雷德（弗兰西斯·麦克多蒙德 Frances McDormand 饰）的女儿在外出时惨遭奸杀，米尔德雷德和丈夫查理（约翰·哈克斯 John Hawkes 饰）之间的婚姻因此走到了尽头，如今，她同儿子罗比（卢卡斯·赫奇斯 Lucas Hedges饰）过着相依为命的生活。一晃眼几个月过去了，案件仍然没有告破预兆，而警方似乎早已经将注意力从案子上转移了开来。 被绝望和
