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 seemingly coincidental series of encounters proves troubling, a horrifying secret from their past is uncovered after nearly 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?