Digimon Frontier (デジモンフロンティア Dejimon Furontia), known as "Digimon 4" in South America, is the fourth Digimon TV series, first broadcast in 2002. After prompted to do so by unusual phone messages, five children go to a subway station and take a train to the Digital World. There, they meet Bokomon and Neemon, who act as their guides and tell the children that the Digital World is being destroyed, and the children must fight the evil Cherubimon and his Legendary Warrior servants in order to save the world.
The biggest feature introduced in the season is "Spirit Evolution", where the DigiDestined themselves evolve into Digimon to become Legendary Warriors, rather than having partners. For this special kind of evolution, they use Spirits, artifacts created by the Ten Legendary Warriors that were hidden in certain areas of the Digital World. When the characters first arrive in the Digital World, their cell phones turn into digivices called D-Tectors. Each character actually acquires two Spirits each, the "Human Spirits" and the stronger "Beast Spirits," and some transform into even stronger forms by being able to use their two Spirits at once and ultimately by combining all the team's spirits together. Another detail different from the previous series lies in the way the characters travel within the regions of the Digital World: most areas are connected by railroads, where train Digimon called "Trailmon" act as a means of transport.
When the five DigiDestined arrive in the Digital World, they find themselves hunted by five other Legendary Warriors who serve the evil Cherubimon. After they defeat four of the enemy, Koji discovers that Duskmon, the fifth evil Legendary Warrior, is really his estranged brother Koichi. With Takuya's help, Koji is able to free his brother, and Koichi restores the Spirits of Darkness to their true forms to join the team as the true Warrior of Darkness, Loweemon.
After they defeat Cherubimon, the DigiDestined learn that the true mastermind is Lucemon, an ancient and corrupt angel Digimon whom the Ten Legendary Warriors sealed away. The six DigiDestined must defeat his two Royal Knights before they scan away all of the Digital World, which would allow Lucemon to be free. Despite their efforts, Lucemon is freed and, after absorbing the Knights, digivolves into a stronger, more powerful form. The DigiDestined combine their power into a new ultimate warrior called Susanoomon, finally defeating Lucemon to save both the Digital World and the Real World.