DigiDestined are a group of children and their partner Digimon who have been chosen to protect the Digital World from various forces of evil. Each child is given a device known as a "Digivice" which serves many purposes but is mostly used to allow their Digimon partners to Digivolve into stronger forms.
Before Digimon Adventure
Prior to the events of first part of Digimon: The Movie, having lost their memories of that event, the eight members of the first DigiDestined group witnessed the battle between a Red Greymon and Parrotmon at their childhood town of Highton View Terrace prior to moving to Odaiba. They were also witnessed by a group of figures who created the DigiDestined Digimon as part of a project to control the balance between good and evil in the Digital World, each Digimon linked to a Digivice and Crest which are connected to the strongest trait of their respective child. However, when the facility came under attack, Gennai managed to get only seven of the Digi-Eggs to File Island while the eighth Digi-Egg, which eventually became Gatomon, as left behind on the Server continent along with the seven of the Crest stones.
Four years later, whilst attending summer camp on August in 1999, Tai, Sora, Joe, T.K., Matt, Izzy, and Mimi are transported to File Island after receiving their Digivices and then meeting their Digimon partners. While they searched for a way home, the DigiDestined eventually learn of their significance in saving the Digital World as they are led to Server to obtain the Crests to enable their Digimon to assume their ultimate level forms. But when the DigiDestined encounter Myotismon, learning time in the Digital World is faster than the human world, it caused a series of events that briefly allow the seven children to return to Tokyo where they are joined by their eighth member, Tai's sister Kari, and Gatomon once she remembers who she is. The event allow enable Tai and Matt is enable their partners to assume Mega form, making them effective against the Dark Masters and then Apocalymon after they practically destroyed the previous Digital World. Though Apocalymon's defeat allowed the Digital World to return to its original state, causing its time to run in sync with real world time, the DigiDestined end up returning to their world. However a series of events, such as an attack on cyberspace by Diaboromon that had WarGreymon and MetalGarurumon combine into Omnimon, allowed the DigiDestined to reunite with their partners again.
Digimon Adventure 02
In the year 2002, with the older members now preoccupied in middle school affairs, T.K and Kari join up with a new group of DigiDestined during the events of Digimon Adventure 02. The two senior DigiDestined having their Digivices upgraded into D3s, which their new teammates use, they and those among their old team who are available at the time act to stop the actions of Ken Ichijouji from enslaving Digimon though his Black Rings and digivolution-negating Control Spires. The new DigiDestined overcome this disadvantage by using Digi-Eggs, each based on one of the eight Crests, that allow their partner Digimon to "armor Digivolve". Though Ken eventually joins their group, it turns out he was influenced by Yukio Oikawa as he sends his creations she and Mummymon to take over what is later to be revealed to be disrupting the Digital World's natural order. As the DigiDestined use new form of Digivolution known as "DNA Digivolution" to merge their Champion-level Digimon into stronger ultimate forms, followed by Paildramon (ExVeemon and Stingmon's ultimate form through DNA Digivolution) gaining the ability to become the mode-changing Imperialdramon through a power boost from one of Azulongmon's digicores, the group eventually learn that Yukio's actions are actually manipulated by an incorporeal Myotismon who eventually regains physical form before being defeated for good.
Digimon Adventure tri.
In the year 2005, with T.K. and Kari now middle schoolers as their older brothers and their friends now attend high school, the original eight DigiDestined reunite in the Digimon Adventure tri. film series after their four newer members have mysteriously disappeared after being defeated by the mysterious Alphamon. While investigating a mysterious infection that turns Digimon rogue, Tai and the others welcome another DigiDestined, Meiko Mochizuki, and her partner Meicoomon. However, they discover that Meicoomon is the source of the infection and to save both the Digital and Human worlds from ruin, the Digital World is rebooted.
In the occasion, almost all Digimon including Agumon and the others lose all their previous memories leaving their human friends devastated. Despite that, Tai and co. travel to the rebooted Digital World where they reunite with their partners and discover a plot by King Drasil, one of the gods of the Digital World, who intends to use Meicoomon, who still retain her memories, as a tool to separate humans and Digimon forever. To this end, King Drasil corrupts Dark Gennai, who betrays the DigiDestined and cooperates with its plan. During the course of the series, not only Agumon and Gabumon regain the power to reach the Mega level and DNA Digivolve into Omnimon, but all the other Digimon partners attain the power to Digivolve to the Mega level as well.