Emily and Michael Stewart are precocious 13-year-old twins, wandering aimlessly around their family's Philadelphia estate during the quiet summer of 1925. Emily has an odd physical tic—she secretly cracks a joint in her ankle and the sound appears to burst through the stillness of midair. Emily and Michael gather the neighbourhood children to fool them with these ‘spirit knockings’. But their game soon 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, their game spins wildly out of control.
