The story follows Future Trunks soon after the defeat of Cell in his own timeline, becoming a Time Patroller and assisting Chronoa in confronting the Time Breakers, later known as the Dark Empire Forces from the Demon Realm. The Dark Empire having revived their leader, Mechikabura, plan to retrieve artifacts known as the Dark Dragon Balls which have scattered across time and space, in order to restore him back into the prime of his youth. However the Dark Dragon Balls have merged with past villains, giving them new found might. Now it is up to Future Trunks, along with new allies such as Goku and Vegeta from stopping their plans from coming to fruition.

(Source: Dragon Ball Wiki)