After the events of Broly: Second Coming, it is revealed that Broly, indeed survived. His body exploding into the sun opened a dimension rift that carried him to a far away planet. He was found by the the kind aliens and nursed him back to health. Broly suffered trama to the head, so he had lost his memories.
Broly lived with the aliens peacefully, and became a focused, sane fighter. He mastered the Super Saiyan transformations and after a unknown time period, Broly suddenly remembed his past as the Legendary Super Saiyan.
Being peaceful now, he controlled his rage and left the planet in search for his enemy, Goku.
Some time later, Gohan, who is now a teacher a Orange Star High School, senses a great power landing near the city. Gohan dismisses class and secretly flies to the site, where Broly is found. Gohan instinctly powers up to his Mystic Mode, but Broly doesn't fight him head on.
It was only after Gohan kept him at bay, that Broly decided to take care of Gohan. Broly transformed into a Super Saiyan and pummeled Gohan with his strength, and his wits. Using his wisdom, Broly hit Gohan in his pressure points, blocking his power. As Gohan is about to meet his end, a angry Goten appeared and saved his brother from certain death.
Gohan and Goten then team attack Broly, and hold him off for a while. Broly then flies into the sky and fires of two Eraser Cannon attacks. As the two heroes go after the attacks, Broly makes his move and flies to Mount Paozu, intent on killing Goku in battle.