Clannad is a story about Tomoya Okazaki, a third year high school student whose mother died when he was young. He grew up with severe drinking problems caused by his father's violent temper, leading to mental and physical injuries. However, his life takes a turn when he meets Nagisa Furukawa, a soft-spoken girl who gets sick easily. Tomoya starts helping her reform the drama club she failed to join last year, changing his view on life.
The game is a visual novel where the player's choices influence the story through occasional "decision points". The game has two parts: the first part describes Tomoya's journey and the second part shows the lives of the main characters several years later. To access the second part, the player must replay the entire game, making different choices each time to alter the events.