What is the plot of Japanese television series Dengeki Sentai Changeman?

The story begins in the year 1990. The five members of the original Changeman are introduced: Change Pegasus (Captain Hayate Yagami/Change Pegasus), Change Griffin (Youma Okutani/Change Griffin), Change Mermaid (Sayaka Nagisa/Change Mermaid), Change Dragon (Show Hayami/Change Dragon), and Change Phoenix (Mai Tsubasa/Change Phoenix). They are part of a defense force and are summoned by the Earth Force's supreme commander to combat the Gozma, an evil insect-like race of aliens who have invaded Earth and are determined to turn it into their new homeworld.

The Gozma are led by the four Heavenly Kings: Basco ta Jolokia (the leader), Giluke (a master strategist), Gyodai and Buuba (twin demons). They are also assisted by the evil priest Ahames. The Changeman are given special powers and equipment by the Earth Force, including the ability to transform into powerful superheroes and use their weapons and mecha to battle the Gozma.

As the series progresses, the Changeman face many challenges and setbacks, but they always manage to overcome them. They also learn more about the Gozma and their plans, and they come to realize that the Gozma are not as powerful as they seem and are actually quite vulnerable. In the end, the Changeman are victorious and they defeat the Gozma, saving Earth from their evil plans.