A huge space monster appears in a city and begins destroying everything. People flee in panic, including five high school students: En, Kai, Shin, Yoku, and Ryu. Unable to escape, they rush into a nearby parked patrol car. As the monster approaches and they resign themselves to death, the patrol car glows blue-white and vanishes with them into a mysterious space. Astonished, they find themselves facing an alien—a Bravian, an officer of the Space Police Organization responsible for universal peace and order. He explains that ferocious aliens from the Space Prison Sargasso are targeting him, and he has given them a battle machine called the "Dag Beagle," built using Earth's copying machines, to fight Sargasso.
The five transform into Dagwon through the Bravian's power and battle space monsters. The patrol car appears before En; he merges with it, becoming the Brave Robot Dag Fire, and defeats the enemy. However, the struggle between the Brave Dagwon and the criminal aliens of Sargasso has only just begun.