Takato frantically has a talk with Ruki on the phone, coming dangerously close to spilling the fact that he and the others are trying to throw a surprise birthday party for her. Ruki, who’s at a train station, is smart enough to figure it out, but as she’s pointing out how obvious it is a train suddenly speeds by her without warning. Ruki decides to run off and check it out.The news begins to report the runaway train, and Takato decides to go after it too while Jian, Shaochung, Terriermon, and Lopmon find their subway train stopped. The other Tamers, in turn, find news of the disturbance spreading around and become curious.Takato makes an attempt to stop the train by having Guilmon evolve to Growmon. He identifies the train as a Digimon called Locomon, and when Growmon questions Locomon on his intentions, he only replies that he has to keep running. Growmon fails to hold Locomon back, and Takato boards the train while telling Growmon to inform the others.Takato walks around Locomon’s cars, deciding that he needs to go to the engine room in order to stop him. Ruki makes her way to a train station that Locomon hasn’t passed by yet, not sure exactly what she wants to do but hoping that she’ll figure out a way to stop it. Meanwhile, while Jian and Shaochung try to find a train station to get to Shinjuku when many of the lines have been blocked, they find a Digital Zone in the sky in Ichigaya. Knowing that something is wrong, they take a detour.The railway officials aren’t sure what to do about the whole situation, but Yamaki decides to provide an intervention, telling them that he’ll be taking control of the situation.Hirokazu and Guardromon meet up with Kenta and MarineAngemon, after which they’re found by an exhausted Guilmon. Guilmon tells them about the situation, while Takato continues to make his way to the engine. Ruki and Renamon find Locomon approaching and jump onto him, finding Takato in the process. Since Takato can’t use it on Guilmon, he gives Ruki a card to use, a Shellmon card that gives Renamon a water attack. Unfortunately, it fails to cool down the engine, and while they try to find other methods, Ruki berates Takato about the surprise party.Beelzebumon arrives to provide his assistance, but fails to stop Locomon and is blown back. The Digital Zone in Ichigaya begins to warp, and at Hypnos Reika and Megumi report that the barrier between the real world and Digital World is crumbling. Since they might be able to send Locomon back to the Digital World if they send him to Ichigaya, Yamaki orders the officials to route him manually.Hirokazu and Kenta find Jian, and they decide to use Guardromon’s power to push a nearby stopped train in order to pursue Locomon and Takato. Hirokazu notices that Jian is alone with Lopmon, and Jian reveals that when he’d tried to run off, he’d gotten confused and left Terriermon with Shaochung.When the officials hear reports of children being on Locomon, Yamaki realizes what’s going on and calls Ruki and Takato. He tells them that he’s planning to reroute the train, and thus they need to get off it as soon as possible. Takato wants to leave it to Yamaki, but Ruki doesn’t trust that he’ll be able to pull it off and insists on continuing work. However, as she makes her way to the front, she’s suddenly stopped by a mysterious force.When Ruki returns to Takato, something’s off about her, and as she walks towards them she’s suddenly threatened by visions of herself as a child, singing as she’s accompanied by her father. As she replaces her younger self, she initially declares that she hates singing before taking up the song herself. As she gets wrapped up in the vision, however, she sees her father walking away from her, and can only walk away in loneliness. This translates in the real world to Ruki walking around aimlessly before suddenly attacking Takato, only saying that she wants to sing.Guilmon makes it onto Locomon with some skillful digging through the ground, jumping onto the back car, while the train Guardromon is pushing goes on pursuit. Shaochung, waiting at a station with Terriermon, sees Locomon speeding by; Guardromon’s train comes from behind, allowing Jian to switch Lopmon and Terriermon on the way.Renamon is suddenly thrown out of Locomon and onto Guardromon’s train, seriously injured. Takato tries to get Ruki to stop, but she continues to attack him. As Guilmon reunites with Takato, a Digimon reveals itself on Ruki’s back, and Jian uses his Ark to identify it as a Digimon called Parasimon. Terriermon sees another one on Locomon, and Jian realizes that they’re controlling Locomon and Ruki.Despite time running out before the train is rerouted and sent back to the Digital World, Yamaki decides to believe in the children...Guilmon manages to slash the Parasimon off Ruki’s back, causing her to collapse. She nearly falls off the train, but Takato barely manages to catch her. The problem gets worse, however, when Locomon evolves to GranLocomon, and Jian realizes they don’t have much time before it’s sent into the Digital World with Takato, Ruki, and Guilmon on board.Renamon manages to board Renamon, rescuing Ruki. However, as the cars separate, Ruki is captured by Parasimon again and Takato goes in to save her, matrix evolving to Dukemon with Guilmon. Dukemon frees Ruki, handing her back to Renamon, and as Parasimon starts playing Ruki’s song, Dukemon lands a hard blow on him. Parasimon suddenly sends a signal into the Digital Zone, however, causing a large number of Parasimon to Realize from it.Now knowing that Parasimon’s purpose was to drive Locomon to the Digital Zone so that it could invade the real world, Jian and Ruki matrix evolve with Terriermon and Renamon to SaintGalgomon and Sakuyamon. Dukemon, SaintGalgomon, and Sakuyamon fight off the Parasimon, joined in short order by Justimon. Guardromon and MarineAngemon join the fight, with Beelzebumon also helping out once Lopmon becomes threatened by some Parasimon. When the Parasimon drive them into a corner, Takato’s sheer determination causes Dukemon to shift into Crimson Mode, destroying whatever Parasimon there are left.With the Parasimon gone, Locomon returns to the Digital World, and Juri kicks off Ruki’s no-longer-a-surprise birthday party. Everyone, however, seems to be enjoying it more than Ruki is, and when Hirokazu asks Ruki to sing for them during a karaoke session, she takes her leave. Takato goes out to see what’s wrong, but Renamon pulls him back and Ruki is left to reminisce.
- Demoted to Extra: Juri, Shaochung, and Lopmon. For Juri, it's somewhat excusable since she's no longer a Tamer, but Lopmon never gets the honor of evolving to Andiramon to help and is only used for a brief Twin Switch gag. Shaochung is just kind of...there.
- Non-Serial Movie: It’s supposed to take place six months after the defeat of the D-Reaper.
- Continuity Snarl: As stated above, the movie’s canonicity is suspect since it conflicts with the drama CD (written by Konaka), which details that the Digital Zone Takato found in the final episode wasn’t enough to reunite the kids with their partners and that what looked like the most likely option to reunite with the Tamers was building an ark to let the kids explore the Digital World (rather than have the Digimon roam freely in the real world as the movie shows). That said, some fans (including the main fan wiki) have tried to rationalize this by placing “Message in the Packet” between the end of the series and the movie, though this is difficult as “Message in the Packet” is said to take place “several months” after the finale.
- Even before all of this, the movie already does have a number of continuity errors which make its canon status debatable. For a particularly egregious case, Beelzebumon appears in his flawed form, before later appearing as Blast Mode, and in the former appearance, he's riding Behemoth. Behemoth appearing could be handwaved as it having been reconstructed, but the inconsistency in Impmon's evolutions can't be explained away so easily, since he's able to do so without his partners, Ai and Makoto - plus their rekindled partnership isn't even acknowledged.
- Replaced the Theme Tune: Like Battle of Adventurers, the dub uses an instrumental version of the theme song for Digimon Frontier due to it being what Disney had the rights to.
- Theme Tune Cameo: At Ruki’s birthday party, Shaochung sings the opening song “The Biggest Dreamer”, while Ruki’s mother sings the first ending song “My Tomorrow”.