This OVA is a remake of three episodes (1, 11 and 105) from the Gatchaman TV series from 1972.

Part I - The Turtle King
The International Technology Commission's project "The Mantel Plan" is a system for the unification of the World's energy resources. This revolutionary plan is targeted for annihilation by an alien race known as the Galactor.

The malevolent Galactor is behind a wave of global destruction using a monstrous ship that has already decimated a city on its path to systematically destroy the project. The Gatchaman team and their prized ship, the Phoenix, must rise to the rescue! To save the planet, it's five against thousands' odds only one team of specialists can handle - The Gatchaman!

Part II - The Red Specter
The secret undersea Galactor headquarters has been discovered, giving the Gatchaman team the chance to cripple the enemy by destroying the base. New forces led by a mysterious warrior in red will join the Gatchaman team in defeating the Galactor this time around, but are these new forces allies or potential foes? The fate of the world depends on the Gatchaman.

Part III - The Final Countdown
Operation codename: Phoenix. The ultimate battle between the human race and the Gallacter is at hand! The Gatchaman team will have to be careful when they embark on their most perilous mission yet! One move determines if humanity lives or dies. It is a brutal fight to the death and when the battlefield clears, both Gatchaman and the Red Specter will pay the ultimate price.

Source: AnimeNfo

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Alternative version: Kagaku Ninjatai Gatchaman