The movie starts with the Sazer-X playing basketball. All of a sudden a flash of lightning fell on the ground. They headed for it, and discovered a boy. The boy was Riki, who assumed to forget everything.
On the other hand, Remy informs the Sazer-X that a giant monster, 'Mecha Giant Beast Bulgario', had approched the Earth and were destroying buildings. The Sazer-X defeated the monster anyway. But then something flashed into their eyes, Takuto had mistaken it as a Cosmo Capsule. It was, in fact, a Bosquito existed inside Bulgario, which was later explained by Professor Horiguchi.
The Bosquito absorbed the JustiPowers in the Sazer Island, reproduced and disguised themselves into JustiRisers.
The boy Riki was sent by the Space Pirates to steal the Cosmo Capsules. Riki was convinced that the Earthlings were Bosquito and were responsible for the elimination of their planet. Vengeance arose in his mind, so he accepted the proposal made by the Space Pirates.
Later he realizes that the humanity was not Bosquito, and the Space Pirates were the one who deceived him in the hope of getting the Cosmo Capsules.
Bosquito commenced to massacre thousands of people by absorbing them. The attires were the only thing left of the people and were all fallen on the ground. The real JustiRisers couldn't believe this scenario. They tried to transform but was in vain.
National Defence made a space battleship – called 'Interception Battleship Gouten' – to invade the Bosquito. They also sent 'Type-05 GS Assist Robot Yuuhi', but it was soon destroyed. JustiRisers went to the Sazer Island and found Tendou Mio lying senseless. All of them helped get back the JustiPowers. Soon Riki breaks G2 and the Sazer-X were incapable of transforming. Takuto was taken by the Space Pirates and was sent to the Phantom Ship where he saves people from other planets (who were destroyed by the Bosquito), including Riki's mother and father.
Riki now realized the deception of the Space Pirates and so did not handover the Cosmo Capsules to them. Rather he kept G2 with him, and let the Sazer-X transform again. Professor Horiguchi kept a Crystal Slab, which would react to the only one whose heart is full of love, courage and justice. Luckily that person was Riki, who helped the GranSazers to get back their powers. JustiRisers and GranSazers were fighting against thousands Bosquito. Then the Sazer-X came for the aid. They combined their powers and destroyed many of the Bosquito. But quickly the Phantom Ship crashed the Earth and absorbed all the powers of the other Bosquito in the Earth and formed Mommoth Bosquito dragon, which was enormous in size, that even the ChouSeiShin and all other robots were tiny compared to that.
The Mammoth-Bosquito was destroyed by Great-Lio (piloted by Lio-Sazer) using the final move, Howling Crush, which the power of the GranSazers, JustiRisers and Sazer-X combined into the finishing attack to destroy the monster.