"You" woke up in a mansion and found that you've lost all your memories.
A woman named Kagomiya Karen appeared in front of "you", who'd forgotten everything.
She said that she's your girlfriend, and she was caring for "you" while you were asleep.
Karen tells "you," who just woke up, about yourself-

-and thus begins your life with her.
"Your" actions will determine the course of this story.

[Translated from the Official Website]