You and Furina were girlfriends, and it was her birthday! All of the Fontaine (playable-) characters decided to give her a surprise birthday party! But they forgot to tell you-... You were at her house, greeting her happy birthday and all. Soon that slowly turneed to "flirting"... and then to... "Fun"... since Clorinde, Navia, Neuvillette etc, didn't tell you. They walked in on you and Furina.