Simon (Jason Bateman) and Robyn (Rebecca Hall) are a young married couple whose life is going just as planned until a chance encounter with an acquaintance from Simon's high school sends their world into a harrowing tailspin. Simon doesn't recognize Gordo (Joel Edgerton) at first, but after a series of uninvited encounters and mysterious gifts prove troubling, a horrifying secret from the past is uncovered after more than 20 years. As Robyn learns the unsettling truth about what happened between Simon and Gordo, she starts to question: how well do we really know the people closest to us, and are past bygones ever really bygones?