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Oushima is first introduced by as the next person she intends Machiya to film in his snuff films, claiming she will commit suicide in five days. This is facilitated by Oushima herself, who has Machiya film her and the rest of the film club on field trip to the beach, ostensibly to film the next production for their film. Mai volunteers to accompany them on the trip and assist in production.
During their vacation, Oushima seems to attempt to seduce Machiya, intentionally having him film her in a swimsuit and even while naked in a hot spring spa. During their time spent in the spa, the topic of Mai's claim that Oushima will commit suicide come up. Oushima replies that she would never commit suicide as she claims to have a boyfriend - in spite of her apparently attempting to seduce Machiya.
On the day before Oushima is predicted to die, one of the members of the film club, a boy named Hikuchi, is found dead in the woods. After being questioned and ruled out as suspects by the police, Oushima talks with Mashiro, and implies Hikuchi was her boyfriend, and asks him to stop filming her. At that moment, Mai appears and attempts to convince Machiya to keep filming, but Oushima pulls him away along a seaside pathway.
In the woods, Oushima reveals she got a text message from Hikuchi's phone demanding she commit suicide or more of her friends will be murdered. As they walk along the path, they see two more members of the film club, Taniguchi and Ootomo dead, having been thrown from a sea cliff. At this, Machiya reveals that he witnessed and filmed Mai commit a murder once before. Machiya and Oushima also note that Mai has somehow manages to evade detection by the authorities.
After ruling out faking Oushima's death after a lengthy discussion Machiya and Oushima agree to attempt to eliminate or capture Mai, and Machiya goes back to the hotel. After he takes a bath, however, Machiya receives a text telling him to meet with Oushima.
Outside the hotel, Oushima received a message from Mai containing a video of the rest of the film club held captive. The video also shows a girl named Mizuno being raped by two unidentified male accomplices.
Oushima then redoubles her attempts to seduce Machiya, allowing him to grab her breast to show him how fast her heart is beating. Meanwhile, Machiya suspects something is off about the whole scenario, and receives a mysterious call from Mai.
Oushima leads Machiya onto the school roof and positions herself as though about the jump. Just before Mai arrives, Oushima kisses Machiya and confesses her love for him. Machiya then hides behind the door to the roof with a baseball bat, but when Mai arrives with a camcorder, she knows he is there. She warns him not to make any "sudden movements" and urges Oushima to jump.
At that moment, Mai is tackled by a still half-naked Saki Mizuno, claiming to be the lone survivor of Mai's killing spree. Mai, however, throw Mizuno off her, against the fence surrounding the edge of the roof, knocking Oushima off the roof.
Machiya sees "Oushima's" bloodied corpse on the ground, and flies into a rage, and beats Mai apparently to death with the baseball bat. After Machiya kills Mai, two film club members, boys named Takahiro and Moto reveal their presence, and Mizuno reveals that that both they and Mai are members of an illegal snuff film sharing site called Dead Tube, and that they intend to win prize money from the site.
Oushima then walks back on top of the building and reveals that she did not actually fall to her death, but landed safely on an mattress on the floor below, and that she threw a drugged student named Yamada off the building and killed her instead. Oushima also revealed that she killed the other members of the club. Taniguchi and Ootomo even willingly committed suicide in exchange for getting the chance to rape Mizuno. Oushima also reveals that, since Machiya joined the film club, they had intended to make this film. She also stated that she was sexually involved with both Takahiro and Moto, and never really loved him. Oushima encourages Machiya to commit suicide after being told his entire high school life was lie.
But Machiya simply replies "too bad" and reveals that he faked Mai's death using a foam bat and fake blood. Mai gets up and murders Takahiro and Moto. At this point, Oushima begs for her life, undressing and offering Machiya sex in exchange for convincing Mai to spare her. Mai respond that she will spare her life if she licks Machiya's shoes.
Mai, however, is quickly proved to be lying. She instead restrains Oushima and hangs her by the ankles over the edge of the school. Mai then cuts the rope holding her and Oushima falls to her death as Machiya films her.
Despite her seemingly nice behavior, Oushima is actually a sadistic snuff-filmer and Mai's main rival. A pathological liar, she displays signs of psychopathy: lack of remorses and empathy. In fact, she is similar to Mai, but unlike her, does not actually love Takahiro. She is even worse than Mai, as she organized the rape of another club member by three boys, and later betrayed her own allies.
Also, unlike Mai, she can be easily tricked, as Takahiro and Mai successfully killed her after a crooked plan involving the false death of Mai. She is a huge coward: before her death, she appears absolutely terrified and tried to offer Takahiro sex, only to be killed afterwards.