Circadia is a first person perspective adventure game with RPG elements in the battle parts. It features characters voices for some of the game characters. There are 14 main characters: 7 males and 7 females. And only 7 characters will be in the last battle of the game. There are different relations (friends or foes) depending of the answers that the player choices.

The game begins when Hiroki Katayama (the main character of the game) arrives an underwater city and as soon as his plane arrives at the airport he meet the spirit of a mysterious woman. After that he meets Yumi Asakura for the first time. A little later he meets his uncle and he takes him to his apartment.

Since that day his life will change forever. When he arrives at his uncle apartment, at night he meet an strange creature that will be with him all the game, and tells him that the world is in danger of collapse and he must help him to save it. That creature show him the other world and a few minutes later when they return to the human world some strange creatures attacks him and the little creature provide him the power to destroy them.

The next day he meets Yumi Asakura again and go with her to the high school and the adventure will start them.