Set after the events of Kamen Rider Den-O, Ryoutaro and the Den-Liner gang have set up a detective agency called the Den-Liner Police who seek to keep the Imagin that survived the Junction Point in check after an Imagin stole a spare Den-O Pass from Owner while he was training. While pursuing the Pink Rabbit Imagin via car chase, they cross paths with the Horse Fangire as Den-O and the Tarōs battle them, killing the two monsters and not finding the Den-O Pass on either. Soon after, the Den-Liner Police are joined by detective Kazuya Suzuki, who tells them of an "Evil Organization" that is making itself known in the criminal underworld. After R-Ryotaro's "arrest spree", Ryotaro and Kazuya encounter Wataru Kurenai and Shizuka as they are suspiciously digging up items for a new violin varnish. After a small spat with U-Ryotaro and an unintentional possession of Wataru by Momotaros, Ryotaro and Kazuya locate the base of the Evil Organization after Wataru tips them off. There, before being chased out, they find Yuto with the organization's leader, the Imagin Negataros, and a man that Kazuya recognizes as wanted criminal Seiya Kuroki. Acting on his own, Kazuya attempts to arrest Kuroki, only to be ambushed by the Clown Imagin. However, with a drunk Deneb and Naomi as a distraction, Yuto, who has been working incognito, attempts to free Kazuya. Before he can ambush Sarah with a metal bat, he is possessed by Urataros who manages to successfuly woo her and wrest the gun away. Yuto kicks Urataros out as he fights her and the Clown Imagin as Kuroki arrives, with K-Ryotaro providing backup. Negataros takes his leave while activating the detonation device with everyone making it out. At the Den-Liner, after Kazuya apologises for almost getting everyone killed and explaining his reasons to be avenging his father Kazuma, Yuto reveals Negataros's plans. Despite telling them that he's too strong, the Den-Liner Police decide to attack at full strength together. The next day, Negataros begins to set his plan in motion with Kamen Riders Den-O and Zeronos, along with their Imagin partners, standing in his way while Suzuki arrests Kuroki after the Den-Liner takes him and his posse away from the fight. However, Negataros uses the Den-O Pass to become Nega Den-O as the fight begins. As the Tarōs deal with the Fangires, Zeronos battles the Clown Imagin while Den-O dukes it out with Nega Den-O. Nega Den-O easily gains the upper hand against all four Den-O forms, but before he can finish him off, Kamen Rider Kiva arrives to turn the tables as Den-O Climax Form joins him in a Double Rider Kick. The impact forces Negataros out of his Rider form. Refusing to accept defeat, Nega Den-O escapes in the Nega Den-Liner with Den-Liner and Zero-Liner in pursuit. Castle Doran joins the fight as all three giants destroy the Nega Den-Liner, taking out Negataros for good. Soon after, Ryotaro takes Kazuya to 1986 to see Kazuma as he was interviewing Otoya on an investigation, giving the young detective closure. Though they succeeded in their mission to stop Negataros, Momotaros decides to keep the Den-Liner Police going as he considers being a cop a cool thing. After they succeed in cornering a couple of crooks, Deneb attempts to ask for Yuto to join the Den-Liner Police, failing to see the crooks they cornered escape with the Den-Liner Police and Yuto chasing after them