Before the events of Shiishii Syndrome...

One day, Tatsuya accidentally spotted his neighbor, Miki, masturbating in the bushes at a park. Though she was already cumming and peeing at the same time, when she realized Tatsuya was watching her, she came again. After she was done, she cornered Tatsuya. To get even, she grabbed his cock and jerked him off. Afterwards, she licked his cum off her face and was surprised at how good it was. Miki admitted that when she has to hold in her pee, she gets horny and told Tatsuya that he can help her out next time she feels that way. Still hot from what just happened, he said he will...

...and that's how they became sex friends.

[Translated from official website]