Dragon Ball Online Kai is set in Age 1000, exactly 216 years after Goku left the 28th World Martial Arts Tournament to train Uub in Age 784 at the end of Dragon Ball Z. During this period of time, a number of notable events have occurred;
Majin Buu created a wife called Miss Buu, and they soon had a son, thus starting the Majin race on Earth;
Namekians have fled to Earth after the Time Breakers army attacked and destroyed New Namek.
Meanwhile on Earth, many Humans have mixed with the Saiyan race and taken up advanced martial arts; some have trained under Krillin’s revived New Turtle School, Tien Shinhan’s re-established New Crane School, and Goten and Trunks’ new Kikoukenjutsu Sword School; schools made popular due to Gohan’s book, which explained Ki control and martial arts to the public.
King Kai also taught the populace of Earth (Humans and Namekians) the Kaio-ken technique, and Majins have managed to mimic it. Also, what appear to be Yardrat have appeared in the Southern Galaxy, and most recently, a gulf formed in time.
The conflict in the story behind Dragon Ball Online is fueled by the Time Breakers, spearheaded by a villain named Mira, who has also enlisted remnants of the Frieza Force and a human faction known as the Red Pants Army.
The motives of Mira involve obtaining the DNA of legendary figures like Goku and Freiza, and achieves his greatest goals through time travel and mind control.
Mind controlled victims of Mira are characterized by a green gem placed on their forehead, and his minions range anywhere from cyborgs to mind-controlled Great Apes.
After using the Time Machine to defeat the Androids and Cell, Trunks was warned by the Lord of Time for his actions in breaking the law of the universe by time traveling and creating alternative histories which could lead up to the destruction of universe.
He also informed Trunks of Mira’s threat, creating destruction through universe by illegally time traveling. After learning this, Trunks takes up the job as a time patrolman to correct his wrongdoing, and stop Mira. He set off to the past and warned Dende and Piccolo of Mira’s threat.