When Scott and Karen, a struggling married couple, take their daughter, Emma, to a picturesque lake house, they hope they can take the opportunity to put their problems behind them. Scott's even hires a nanny, Gwen, to give them time more to spend together. Gwen does well, working with Emma, but the little girl begins fixating on an imaginary friend she calls the Fish Man. Scott discards the notion, but Karen believes the friend is actually a strange man who lives down the beach named Harry. Emma's fear of the water and her recurring sleepwalking continue to raise tensions in the house and drive a wedge between Scott and Karen until, one night, Emma disappears. Police are called and they search the area, but they're unable to find the little girl. As Scott and Karen worry that Emma is in grave danger, Gwen finds her in a bathtub in the house... seemingly breathing underwater. The family is relieved, but Gwen can't shake the idea that something's wrong. She confronts Emma about where she was all day; Was she with Harry? But Emma insists she was with the Fish Man, and when Gwen says the Fish Man isn't real, Emma tells her that the Harry is dead. And the Fish Man is coming.