The highly marketable conceit of director Tim Wardle’s documentary is that triplets born in 1961 then separated spent the first 18 years of their lives totally ignorant of their siblings. When they reconnect, it’s a joy. But the movie quickly switches gears to explore the question of why they were separated at birth to begin with. It’s that investigation—and the chilling answer—that lends Three Identical Strangers its bittersweet, haunting atmosphere. It’s currently on Hulu.