That day, Shidou felt something strange with the duplicate key he found in his house. The thing that he should have handed it to someone for sure. However, he doesn't remember to whom he should have handed it to. At that moment, a small noise appeared above his head, and takes the consciousness of Shidou.
Upon being awakened, Shidou found himself in a world where Arusu Maria exist, who does not have a physical body in reality. And then, a certain girl appeared before Shido.
Sonogami Rinne ― At one time, a girl that Shidou had failed to save.
Shidou, who regained the memories of when he had known, becomes possible to investigate the cause of the resurgence of Eden .
In the midst of it, who appeared before Shido was, a young girl whose appearance resembles that of ― Rio.
"It's no good if you don't find the most important thing to you."
What does Rio means by "the most important thing"? Is she an existence related to Eden ?
In this closed world, the action should Shido take in order to regain the everyday life is only one―
By dating and make them fall in love!?

[From DAL Wiki]