Synopsis
A high school student named Suhan has trouble getting along with his sister Sunmi. One day, he visited Jinyong's house and saw Jinyong get along well with his sister. Jinyong pulls out a small stone and tells the envious Suhan, "I was not on good terms with my sister either, but this helped us get along."
With the magic of the stone, Suhan was able to change Sunmi's memories one by one, and because of the changed past, he got close to her. After Suhan fell in love with changing the past, things began to happen that never occurred before.
Could you be my little sister?
This is generally referred to as a Visual Novel. (Example: NEKOPARA, Sakura, etc.)
Each time you make a choice at the decision points, the plot changes, resulting in a different ending.
Before these decision points, the prologue portion of the game will conclude.
Purpose
A time travel stone can help you reach a closer sibling relationship with Sunmi.
Depending on how you treat Sunmi when you time travel, your relationship with her will be affected.
Time travel, however, is not always a positive experience.
If you end your time travel at a different time, the ending will be very different.
Until these decision points are reached, the prologue portion of the game will have been completed.