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The Venus Series (ヴィーナスシリーズ, Vīnasu Shirīzu) is a Keijo tournament held at the Suminoe Keijo Stadium. Differently from many Keijo tournaments, the organizers deliver a challenge letter to specific players to invite them to the event, instead of allowing players to sign into the event with certain qualifies.
Saya Kogatana and Nozomi Kaminashi are invited to this event. Saya, however, challenges Nozomi and makes a deal: if she losses, she will stop from using her "Quick-Draw Breast Sword" for her safety, but if she wins, Nozomi and the others must stop from advising her to use butt techniques.
Despite this, Saya and Nozomi are targeted by a group of East players, consisting of Kaoru Oshiri, Rei Mizushima and Tamaki Matsui. Thus, they need to work together and cancel their duel. However, Nozomi is later cornered after protecting Saya, who can no longer use her "Quick-Draw Breast Sword". On the other hand, Saya is ultimately willing to abandon her promise with her mother, and uses the "Beheading Butt Sword", a butt technique advised by Nozomi and the others. Thanks to their teamwork, Saya manages to win the competition.
Apologizing to her mother, Saya wonders if her mother would forgive for what she has done. On the other hand, her uncle satisfies that Saya's friends are able to help her solving the problem. Suddenly, an old lady shouts at them while declaring to be one of the Five Butts, much to the spectators' surprise.