|The Tamers are preparing a surprise birthday party for Ruki. During a phone conversation with Ruki, however, Takato accidentally tells her about the party, annoying her greatly. She is distracted, however, by the sudden appearance of a Locomon, running out of control. Takato and Guilmon, upon encountering the rogue Locomon, attempt to force it to stop, but are unable to overcome Locomon's strength and intense desire to keep running. Takato manages to board Locomon, and begins to investigate it while Guilmon sets out to alert the other Tamers.
Guilmon is found by Hirokazu and Kenta, who are told immediately of Locomon's actions and set out to help bring it to a stop. The Locomon also attracts the attention of Yamaki, who joins the railway's operators as they attempt to prevent Locomon from causing too much damage. Meanwhile, Jenrya and Shaochung, attempting to return to Shinjuku on foot after Locomon's behavior causes the cancellation of their train, notice a Digital Zone. Ruki and Renamon join Takato aboard Locomon to aid in the investgation, but to no avail. Ruki, frustrated, tries to stop Locomon manually, but fails, and returns to Renamon and Takato in a trance-like state. When Takato asks her what is wrong, she begins attacking him. Renamon tries to intervene, but Ruki turns on her and knocks her out of the passenger car. Ruki then proceeds to pursue Takato onto Locomon's roof.
Guilmon, having boarded Locomon, begins attacking Locomon's interior, causing Parasimon to be revealed attached to both Ruki and Locomon. When the Parasimon attached to Rukibegins attacking Takato itself, Guilmon destroys it, freeing Ruki from its control. Almost immediately after, Locomon evolves to Grand Locomon. To exacerbate the matter, several more Parasimon begin emerging from the Digital Zone. Takato, Jenrya, and Ruki Matrix Evolve to Ultimate in an attempt to defeat the incoming Parasimon, and are joined by Justimon. The attempts are unsuccessful until Dukemon Mode Changes to Crimson Mode and manages to destroy the Digital Zone, thus killing the Parasimon.
Afterward, Ruki's birthday party proceeds as planned. Ruki is dissatisfied, however, and leaves as the party continues without her.