Kids have imaginary friends. Rachel knows this. So when her young son, Billy, miraculously recovers from a horrible flu that has proven fatal for many, she thinks nothing of Delfy, his new invisible friend. After all, her family is healthy and that's all that matters. But soon Delfy is telling Billy what to do, and the boy is acting up and lashing out in ways he never has before. As Delfy's influence is growing stranger and more sinister by the day, and rising tensions threaten to tear Rachel's family apart, she clings to one purpose: to protect her children at any cost --- even from themselves.