The hero had one problem...
That's because his little sister Arisa has become terribly unfriendly to him.

They used to have a good brother & sister relationship, but ever since his sister reached puberty, they somehow grew apart.

However, one day, the hero witnesses Arisa's anal masturbation scene.
He decides to download an eroge with a younger sister heroine in an attempt to understand his sister's feelings.

However, a lightning strike suddenly occurred during the DL, and at the same time, a girl jumped out of the PC screen!
She was the main heroine, Arisu, of "The Younger Sister of Wonderland," the eroge that the main character was DLing.
Furthermore, Arisu somehow merges with Arisa during the strike...
As a result, two hearts dwell in one body.

In order to restore them to their proper state, the hero must lead a double life with Arisa & Arisu while attempting to get the situation back to normal.

[Translated from DLsite]