The threat of monster attacks fades, and the Earth seems once again at peace. Humanity turns its eyes to the stars and heads to space, while its monster attack countermeasures continue to shrink.

It is in this new age when "Sphere," an invading swarm of mysterious floating space objects, begin to attack Earth. Humanity loses contact with space and becomes a lone star. Their attack throws our hero Kanata Asumi's peaceful life into chaos. Destruction spreads before his eyes and a giant monster appears. With no heed for his own safety, Kanata jumps in front of the enemy and transforms into Ultraman Decker.