A manga adaptation of the of Toei TV series, published in Bessatsu TV Land in the 1973 March and April issues.
 
(Source: Wikipedia Japan)

From deep within the Earth's surface, the Dinosaur Kingdom make their move to wipe out mankind and conquer the planet with their mechanical dinosaurs. After a prototype Getter Robo is shot down by a mechanical dinosaur, the Saotome Research Institute decides to use the real Getter Robo, which is combat-capable. Due to a lack of pilots in the institute, Ryo Nagare goes to his college to persuade martial artist Musashi Tomoe and outcast Hayato Jin to join him and pilot the Getter Machines to combat the new threat and protect mankind.
 
(Source: Anime News Network)

The first Getter Robo series chronologically. Published 15 years before the rest of the manga, Getter Robo pioneered the idea of the combining and transforming robot. 
Millions of years ago, the dinosaurs were driven deep underground due to the sudden arrival of "Getter Rays" on Earth. After evolving into sentient beings, they grew bitter and vengeful at the humans who replaced them on the surface and began plotting their attack on humanity. 
Fighting against them is Getter Robo, a machine built by Professor Saotome which uses the mysterious Getter Rays as its power source. Piloting it are three strong-willed young men - the violent martial artist Nagare Ryoma, the psychopathic yet highly intelligent revolutionary Jin Hayato, and the hot-blooded Judo master Tomoe Musashi. Each controls one of three specially designed combat jets which can be combined together in different forms to form three different kinds of giant robots: the flying, axe-wielding Getter-1, the quick, drill-wielding Getter-2, and the tough, tank-like Getter-3.

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