Emily Stewart and her brother Michael are 13-year-old twins privileged, precocious, wandering aimlessly around their family's Philadelphia estate during the quiet summer of 1925. One day Emily discovers an odd physical tic she can secretly crack a joint in her ankle so the sound appears to burst through the stillness of midair. Knock, knock ... In their garden tea house, Emily and Michael gather the neighbourhood children to fool them with these 'spirit knockings'. But soon this game of contacting the dead creeps into a world of adults still reeling from World War I. When the twins find themselves dabbling in the uncertain territory of human grief and family secrets knock, knock—-their game spins wildly out of control.