Twenty years after their planet was attacked by the brutal Zone Empire, the five Hoshikawa children, who escaped and were raised on Earth, must prepare for a brand-new war. This time, though, they're ready ... having developed the powerful Fiveman technology that will help them defend Earth from the same empire that conquered their home world.

In 1966, five infant children were kidnapped by an alien group known as the Alien Hunters for the Reconstructive Experiment Empire Mess, who wanted samples of humans from Earth to experiment on. The children were rescued by Mess' arch enemy, the Flash alien race, which took each one to a different planet of the Flash solar system for training. Each child was trained separately in a range of superpowers that will allow them to fight Mess, their bodies also adapted to the atmosphere of the Flash Solar System making them gain special abilities. When they finally return to Earth in 1986 to combat Mess, who is now trying to invade it, they use the opportunity to search for their birth parents.

Big Rally! Titan Boy! is the second movie of Choushinsei Flashman. It is a theatrical edit of episodes 15 to 18 (The Giant Robo is Worn Out, The Human Minimini Plan, The Mysterious Giant Reckless Car! and Big Turnaround! The Transforming Robo) with new and extended scenes.

The Flashman find themselves protecting a puppy targeted by Mess due to it being the weakness of a chimera Beast Warrior.