[Which work of fiction is this about? Is "Martian Manhunter" the title of the work? If so, it should say so at the beginning. Is it a book, a movie, a story serialized in periodicals, or what?]
In the comic book back story, the entire Martian race was wiped out by a mysterious alien force and he was the only survivor. J'onzz came to warn that the same aliens were coming to invade Earth. He met Superman and Batman and they formed the Justice League, 7 of Earth's greatest heroes, to fight off the invaders. With the help of Batman, Superman, Wonder Woman, Green Lantern, The Flash and Hawkgirl, and his own powers including telepathy, super strength, transparency and transformation, they defeated the Alien Invaders. Martian Manhunter now lives at the Justice League Watchtower and defends Earth with the rest of the League.