As it's common in these kind of games, you are playing a young horny Japanese boy named Yosuke.
One day Yosuke wakes up and feels that something has changed inside of him. Some dark spirit has come into his mind and from now on he tries to make sex slaves out of the people who care for him after his parents have died.
Tsuki: Possession is a dark and serious representative of its kind. It can be more likely compared with Virgin Roster than with hentai games like Heart de Roommate. But you don't have to play such an evil character, during the game Yosukes attitude changes and he changes back into a normal person.
Tsuki: Posession is an interactive novel. You play by clicking through the story and from time to time you have to make a choice that affect the following events. Furthermore, the game features multiple endings and lots of CG illustrations.
Because the game is full of sex scenes it should be played only by adults.