Operation: Doodlebug is the name that was given to the final brute-force operation to reduce the D-Reaper entity to its harmless original form.
The plan was developed in a last ditch effort by the Monster Makers team. According to the plan, the Juggernaut program, which had served a less benign role earlier in the series, was to be loaded into Terriermon, who would then, in the form of MegaGargomon (the biomerge digivolution of Terriermon and Henry Wong), enter an opening in the vortex that was created by the D-Reaper to link between the Digital World and the Human World. There, he would proceed to spin in the opposite direction of the vortex's flow, creating a backwards flow in time that, combined with MegaGargomon's release of the Juggernaut algorithm in a "big bang" form, would result in the forceful reversion of the D-Reaper back into the Digital World and back into the form that was less complex than a calculator, as was originally intended.
However, as this operation was accomplished, the Monster Makers realized, too late, that the effect of the combined time reversal and Juggernaut implementation would not only return the D-Reaper into its original form, but would also force the Digimon in the human world—including the Digimon partners of the DigiDestined—back into increasingly-regressive forms. Thus, in order to be saved from the total deconstitution of their false proteins and data, the Digimon in the Human World had to return to the Digital World, which, like the Human World after it, was recovering and rebuilding from the death and destruction wreaked by the overactive D-Reaper program.