秘密の放課後 (Himitsu no Houkago)

Background
VOCALOID idol debut at last!

"Himitsu no Houkago" is a more risque VOCALOID work with an idol atmosphere and a dating sim theme. natsu-P originally uploaded it to Niconico under the username "？？？" due to the song's lewd nature. She has since revealed herself as the producer, and it has since become one of her more notable works.

The song features nine female VOCALOIDs (Iroha, Miku, Luka, Miki, Lily, Meiko, Yuki, Rin and GUMI) and rap segments in Vietnamese using Gackpo suspected to be sampled from from the Vietnamese song ("Mắt nai cha cha cha"). The nine schoolgirls fight for the attention of a boy and gradually have more and more intimate encounters as the song progresses in secret afterschool. It is debatable how sexual they are.

This song has been featured along side the sequel in the E.P. album 秘密の課外授業!.