The game takes place two years after the defeat of Majin Buu, during a time period where monsters are taking over cities and destroying everything.
However, an event more powerful than all other monster attacks is about to take place... It is revealed that there are three mysterious beings called "Time Breakers", who plan on traveling through time in order to change events in history for their benefit. First, they travel to Age 774 with the intent of killing Goku before he can reach the battlefield where Vegeta later dies. Super Saiyan Future Gohan arrives too late and finds Goku's dead body.
Two years later, at age 776, Future Trunks has given up fighting crime because he feels guilty for letting everyone down when his father died