Stephen (Colin Farrell) is an excellent cardiologist. He and his wife Anna (Nicole Kidman), who is also a doctor, have two children. Their family is very wealthy and happy. Stephen recently became friends with a 16-year-old boy, Martin. The two talk about everything and are very close, just like father and son. Stephen often invites Martin to his home as a guest, hoping that Martin, who often seems to be worried, can become friends with the children and cheer him up. However, the daily life that should be harmonious has changed because of Martin's entry. A series of strange things and unexplainable diseases have occurred one after another. An unspeakable strangeness and fear began to shroud this family. What is Martin's true face? How can Stephen save this gradually eroded family life?