Based on: The Justice League episode "Secret Origins"
The film opens on Mars during the Cretaceous Period (65 million years ago) on Earth. J'onn J'onzz, his band of Manhunters, and a few women and children are the last of the Green Martians. After a 7 year war over Martian resources and religious differences, the White Martians have put the Green Martian race on the brink of extinction. Due to the war, Mars is becoming less and less inhabitable. We see J'onn and his squad leave their families to attack a secret White Martian project. Once the Manhunters arrive at the White Martian base, they discover--to their horror--that the White Martians intend on leaving for Earth on two ships and destroying all life on Mars with an "Extinction Bomb." J'onn and his brother split their band into two groups to tackle this issue. J'onn's brother and his squad sneak aboard the first White Martian ship headed to Earth and destroy it while it's on its way to Earth. The resulting debris wipes out the dinosaurs. After his squad dies fighthing, J'onn redirect's the "Extinction Bomb" to the second White Martian ship headed to Earth. The explosion damages the White Martian ship, kills all Green Martians remaining, traps the White Martian ship underneath the surface of Mars, and leaves J'onn and hundreds of White Martians trapped in the ship's suspended animation.
65 million years later, two astronauts land on Mars to investigate radiation being emitted from underneath the surface of Mars. They fall into a Martian cavern and accidentally free the Martians from suspended animation. One is swiftly killed, the other--John Jones--is wounded as J'onn helps him escape back to his ship. John Jones dies on the journey back to Earth after he asks J'onn to help the people of Earth. J'onn assumes his identity.
We then jump to three months later. "John Jones" pays a visit to Star Labs to meet with Tech Titan Victor Stone. Dubbed "Cyborg" by the press, Victor Stone and his team of scientists travel the world discovering and reverse engineering alien technology that has crash-landed on Earth. On one of his expeditions, he suffered a near fatal injury, but was saved when the "Omegadrome" bonded to his body.J'onn reveals his true form to Cyborg, and he psychically shows Victor what is coming.
On Paradise Island, Diana of Themyscira has visions of the end of Man's world. She pleads with her mother to leave the magic veil that protects Themyscira so that she can help protect the world. Her mother refuses and tells her that if an Amazon leaves Paradise Island, she may never come back. In the cover of darkness, Diana follows her heart and leaves the island. Green Lantern Hal Jordan picks up big traces of energy headed to Earth. When he goes to investigate, he is overwhelmed by Martian forces and is forced to head back to Earth to recharge his ring. The Martian forces are not too far behind, and a full scale invasion of Earth commences. Despite being warned by Cyborg and the Martian Manhunter, Earth's armies are devastated by the Martian Vanguard. Cyborg and Martian Manhunter are captured and held captive in Martian sciencells. The Martians plan on studying them before killing them. In their cells, the two meet other captured heroes who tried fighting the invasion--Superman, Wonder Woman, The Flash, and Green Lantern. The six manage to escape captivity after the Flash speeds up his heart beat to the point that it looks like he has flatlined. When the White Martians come to check on him, the six make their move.
Superman quickly establishes that he is the natural leader of the group when he inspires the group to work together. They head to Gotham City to take the fight to the epicenter of the Martian invasion. There they encounter the Batman, who helps them to plan an attack on the Martians. In the end, the team is victorious. A few days later, Superman convinces the group to stay together because they can do more as a team than as individuals. "What, so you want us top be a bunch of Super Friends?," says the Flash. Superman replies, "No, I was thinking more like... a Justice League."
Justice League II
Based on the story "Tower of Babel" by Mark Waid
More to come...
Justice League III
Based on Season 2 of Justice League Unlimited
More to come...
Justice League IV
Based on Season 3 of Justice League Unlimited and The Once and A Future Thing
More to come...