Ajisai no Chiru Koro ni episode 1

Yuta Yamano had a gilfreind whose also his childhood friend , her name is Natsuha Shinomiya
They attends the same school , this summer….hes really worried about her because she turned strangely aggressive lately.
His anxious feeling was right , Natsuha wasn’t satisfied with Yuta , She secretly invited her famous “yarichin” classmate to her place and doing it.
i.e yarichin means people who had sex with many women.