There is a cabin in the woods that serves as a laboratory and a dwelling for a Lodger. Lately the Lodger experiences strange changes in the surroundings - each night some things get missing, noises are heard and mysterious guests are coming from the woods. The Lodger is insomniac and he needs to stay awake and sane till dawn. He also has lost something important, something that may be the reason of all the strange events.
At its heart Knock-knock! is a hide-and-seek game. Each level (or night) is timed and all you need to do is wait till the morning comes. The way you do it is up to you, the player. You may roam around the house and investigate what's going on - turning on the lights and unlocking doors. You may choose to "fill in the gaps" - make certain things appear in the lit rooms. When bizarre "guests" start appearing in your house you may try to close the "portals" from which they come or you may choose to hide behind the bed and hope they don't spot you. From time to time you will leave your dwelling and find yourself in the woods or in a seemingly endless corridor. The story of the Lodger is told in strange messages that you find and by the Lodger himself at certain moments. At some points the rules of the game may slightly change and it is up to the player to understand what has changed and why. Some of your actions will determine the outcome of the story and alter your understanding of what's going on.