The protagonist parts ways with Washiba Yayoi, who he met on vacation, and gets on another train.
When he arrives home news of his grandfather’s death, and will, are waiting for him.
His will states “Your inheritance is to succeed the Washiba estate in its entirety. Under the condition that you marry one of the women of the Washiba family and live together in the mansion with them.”
Now then, what will he decide?
[Translation of synopsis from Getchu.com]
