I'm not sure specifically as there could be many novels with this title. It might be about the anticipation and mystery that comes with a knock at the door. Maybe it's a horror story where the knock at the door signals the arrival of something sinister, or it could be a romantic encounter that begins with a knock.
It often deals with mystery and suspense. You never know who's on the other side of that door and what it might lead to.
I'm afraid I can't say who the main characters are without more information about this 'a knock at the door novel'. However, it's common in such stories to have a protagonist who is affected by the knock at the door. There could also be the person or entity on the other side of the door. If it's a thriller, the main character might be a detective or an ordinary person caught up in a dangerous situation that starts with the knock.
There's a chance it could be a mysterious stranger. They might be on a journey and need a place to stay for the night. Or they could be bringing a warning or a message that will change the life of the person inside the house.
Once upon a time, in an old, creaky house, Lisa was all alone. It was a dark and stormy night. Suddenly, there was a loud knock on the door. Her heart started pounding. She slowly walked to the door and peeked through the peephole but saw nothing. The knocking continued. She gathered all her courage and opened the door, only to find a branch from a nearby tree hitting the door because of the strong wind. It was a relief but also a spooky moment.
One possible theme could be the unexpected. A knock on the door is often an unanticipated event, so the story might explore how characters deal with the sudden intrusion into their lives.
One stormy night, an old man was sitting in his creaky old house. A knock at the door startled him. He opened it a crack, but there was no one there. As he was about to close it, a cold hand pushed the door open further. He saw a figure in a tattered white gown, its face hidden in the shadows. The figure just stood there, not moving or speaking. The old man tried to push the door closed again, but it was as if an unseen force was holding it open. Then he heard a low, moaning sound coming from the figure, and he fainted from fear.
Maybe it could be your grandma. Grandmas are often great storytellers. They have a wealth of life experiences and a bunch of interesting stories to share. When she knocks at the door, she might sit you down and start with a fairy tale from her childhood, like Cinderella or Snow White.
Well, it could be a story about a lonely person living in an old house. When they hear the knock, they open the door to find a lost child seeking help. This then leads to a heart - warming journey of the person helping the child find their way back home.
One common element is the unknown. When you don't know who or what is knocking, it creates a sense of fear. For example, in many stories, like the one where the girl is home alone and hears the knock, she has no idea what's out there. Another element is the isolation. If the character is alone, like the old man in his house, it makes the situation scarier. And often there are strange noises associated with the knock, such as scratching or moaning, which add to the horror.
It could also be a story of unexpected opportunity. The person who hears the knock is in a tough situation in life, like being unemployed or in a bad relationship. When they open the door, there is a stranger who offers them a job or some life - changing advice, thus starting a new chapter in their life.