Title Etymology:
Kasumi is described as a "black-hearted type school idol" (腹黒系スクールアイドル, haraguro-kei sukūru aidoru), while Shizuku is described as a "junior type school idol" (後輩系スクールアイドル, kōhai-kei sukūru aidoru).
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In the past, at Nijigasaki Academy, theater fan Shizuku Osaka becomes interested in the recently made School Idol Club. After finding the room, she is initially confused to find no one inside, only to encounter one member, Kasumi Nakasu, who had just arrived.
Some time later, Shizuku, now a member of Royston Slinger's gang, has decided to visit the grave of a former Digidestined, with 5 other girls at her side. The 5, Lanzhu Zhong, Mia, Taylor, Shioriko Mifune, and two others in black masks, are figuring out what to do about Team Arch, now that they know almost everything about Slinger's organization, but Shizuku suggests a "special trap" for them.
In the present, Honoka Kōsaka is planning on a big vacation for her family, along with other idols and their families. She even invites Takuto, but he isn't sure if he we wants to go, considering that Honoka has also invited her cousin, Shinra Asakura, who is also his childhood frenemy, as the two used to fight over Kotori.
Meanwhile, Team Arch is still looking for a new partner in Sacramento, with only mixed results, and it becomes more difficult when they respond to three false calls, set up by three different assailants, overwhelming them. As they were about start another day, Hanamaru brings in two sisters, Dia and Ruby Kurosawa, who also happen to be school idols from Uchiura. The two wish to help with Team Arch's problem, in exchange for membership in Team Arch, which SpongeBob agrees to.
Meanwhile, in the city, Haruka Konoe, a 1st Year from Odaiba, had come to Sacramento to visit her sister, Kanata, in an effort to have her lesser-known school idol group, who had recently transferred to Nijigasaki, merge with the Nijigasaki group. Just as Haruka was waiting for the bus bound for Kanata's apartment, Shizuku, who is relatively unknown to Haruka, approaches her, warning her to stay away from Kasumi. Confused, Haruka hops onboard the arriving bus, wondering why Shizuku wants her to stay away from Kasumi.
Back with Team Arch, Malcom Coleman is at the practice range, practicing his marksmanship out of boredom. He was about to finish up when Tyson Gallego, a close friend of Team Arch, and an ex-SFPD officer, arrives looking for a job, having heard of Team Arch's relocation. Malcom offers Tyson a job in the Arch Foundation, but Tyson wants to remain a supportive officer instead. Suddenly, Kanata comes in after receiving a call from Haruka (who has discovered the apartment to be empty after arriving) about Shizuku warning her about Kasumi. Just they were about to check on her, gunshots are heard at César Chávez Plaza, prompting the team to head over.
At the plaza, the situation becomes to heated for the Sacramento Police Department and even their own SWAT team, prompting them to call in the California Bureau of Investigation and their SWAT team to help. Team Arch arrives to help, setting up a makeshift fortress out of vehicles and a sniper locator. The team manages to save the only injured person, but the sniper destroys the locator, inadvertently revealing the location, The Citizen Hotel. CBI has every other possible sniper location on lockdown, while Austin MacGyver leads his team to the roof of the hotel, only to find it clear, with only a "kevlar-breaker" bullet left behind, prompting the teams to use shields.
Meanwhile, a giant black bird flies over the plaza and lands on top of City Hall's Parking Garage, with the people down below brushing it off, unaware that the black bird is actually one of the 5 girls with a morphing ability bringing herself and four of the girls to the rooftop. Back on the ground, a female officer arrives at the garage to set up a sniper's nest of her own to fight back. However, as she arrives to the roof, she meets up with the other 5 and pulls out a sniper rifle, revealing one of themselves to be the mysterious sniper.
