In 1970, Doctor Hoshikawa was researching how to transform the planet Sedon into a green, lush world, testing it by attempting to grow flowers. On the day the first flower bloomed, the Zone Empire launched an assault on the planet, he and his wife were separated from their five children. Arthur G6 took the five children back to Earth and raised them.

Twenty years later, the five are now teachers in the same school. The Zone Empire now prepares to invade Earth as its thousandth target and as they begin the attack, three vehicles appear and counter the offensive, five warriors descend from the vehicles and confront Zone.

The Hoshikawa siblings have been developing the Fiveman technology and training hard upon the possibility of Zone invading Earth. Now the five siblings are ready to battle with the familiar foes as Fiveman.

After defeating the Zone Empire, the Fivemen departed Earth to recover their stranded parents.