5 children receive strange messages on their cell phones that lead them to a train that takes them to the Digital World; a strange world filled with bizzare creatures called Digimon. These children have been sent there to stop an evil Digimon named Cherubimon from completely annihilating the planet of all its inhabitants. In order to accomplish this, the 5 children must locate their spirits, which will evolve them into the Legendary Warrior Digimon. Unfortunately for them Cherubimon has ordered his servants to stop the kids from finishing the mission; eventually the children will have to fight Cherubimon's forces in order to save the planet.

With five new kids and an exciting new mission in the Digi-World, Digimon Frontier brings back all the great action and adventure of the last three seasons. Takuya, Kouji, Zoe, J.P. and Tommy meet each other in a train that takes them to the Digi-World where a war against evil is being fought. The Angel digimon, Cherubimon, one of The three angels sent to save the World from the power-hungry Lucemon, has turned to the dark side and the entire Digi-World is in peril. To fight this great battle, the five chosen ones must become digimon. Something that no digidestined has done before.

Source: ANN

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Same setting: Digimon Adventure
Side story: Digimon Frontier: Ornismon Fukkatsu!!