This is the question that continues to haunt disgraced police officer Ali Davenport, fifteen months after the devastating case that changed the course of her life.

Now, after the death of her former colleague, Ernie, Ali has returned home to face a past that won't stay buried. Found in the disused office building where he worked as night security, Ernie's death has been ruled as suicide. But not everyone is convinced. Wild stories are circulating about a supernatural presence in the building, an entity that might have attacked Ernie that fateful night.

With the sale of the building about to go through, Ali is hired by its owner to work Ernie's remaining night shifts and debunk the potentially damaging story. An easy enough job, if you don't believe in ghosts. But then Ali meets Will, a teenage ghost hunter who claims to have evidence on film...

Ali and Will soon fall headlong into a mystery that takes them through their town's macabre history and into Ali's own dark past. As the pieces of the puzzle come together, Ali is forced to face the question of The Knock-Knock Man. What she doesn't know is The Knock-Knock Man has already been watching her for a very long time...