Hikari and Tailmon.Taichi's group dive into the ocean on their way to FAGA, the location of Millenniumon's largest piece, to prevent his resurrection, only to be knocked off course by the ocean current and take shelter in an underwater city led by MarineAngemon. After spending the night in the city, the children are about to resume their journey when the city is attacked by Anomalocarimon and a group of Mantaraymon. While the other Digimon of the group fight alongside their human partners, Tailmon insists on fighting alone to protect Hikari, until Hikari convinces her that they must fight together and Hikari gains her own Digivice, using it to power up Tailmon who leads the others to defeat the invaders...