November 1950. You're a journalist specializing in occult sciences and paranormal phenomena. After receiving a strange telegram signed by "the Count" inviting you to a party in a mansion in the countryside, you drive there, only to find the mansion empty. Now the entrance doors have been locked behind you, and you must fight your way into the house, find weapons and clues and survive until midnight.
Invitation is an adventure game with light RPG elements. The player must explore the rooms of the mansion, fight the creatures and find items and clues. The player has two statistics to manage, physical health and mental health. The game ends when either one drops to zero. Items include weapons, items allowing to destroy instantly a certain type of monster instead of fighting it and codebooks allowing to decipher the clues in the game manual (encoded with a simple alphabet-based cypher). The clues help the player figure out which items are deadly to some creatures, and what needs to be done to finish the game.