New Million, "the city where a million dreams come true," has developed thanks to the efforts of its Heroes. In this city, visited by people from all over the world, a dome-shaped stadium floating on the sea has been born. It's called "Grand Sea New Million." Plans to hold a summer festival as the opening event have been floated, and the stadium management has asked the Heroes for their help. [HELIOS] has agreed to fully cooperate for the sake of the city. Not only will the Heroes perform at the festival, but they'll also be helping with the event itself, working across sector boundaries to prepare for the festival's opening. However, one problem after another arises... Furthermore, behind the scenes, [Eclipse] and his allies are secretly plotting. Will the "Summer Festival" be able to open safely?