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 even hires a nanny, Gwen, to give them more time 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. 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.