The story takes place on a holiday after Rin got together with Kaede.

Primula comes back to the Fuyou home as a family.
The King of Gods and King of the Devils examine Primula's emotional control in front of Rin and Kaede. However, right in the middle, Primula's magical force - which far exceeded their expectations - goes out of control and affects Kaede's mind.

The various memories within Kaede are rearranged in tatters.
As a result, Fuyou Kaede is left in a state of utter confusion and falls asleep.
Nobody knows when or if she will awaken.

Rin infiltrates Kaede's mental world with his friends and tries to correct the scattered memories to save her.

[From MangaGamer]

Really? Really! is the third installment of the Shuffle! series. It is a continuation of Kaede's path from the first Shuffle! visual novel.

The player must use keywords acquired from viewing various events in Kaede's memories to fix the memories.

[Edited from Wikipedia]