Disclaimer: Knowing full well there is a wealth of resources available to fans of the "Justice League" animated series, I have no intention of doing a bunch of dry story synopsis with the occasional new screen grab. I will chronicle, within reason, J'Onn J'Onzz's specific journey over the course of the series, but chiefly I will be reviewing the episodes through my own jaundiced perceptions.
On Themyscira, Princess Diana makes off with the armaments of Wonder Woman like a thief in the night. Shoddy origin? No contest.
Outside the base, the "military" turned alien strike force attacks J'Onn J'Onzz and the World's Finest. The Martian wraith passes through the Dark Knight, then solidifies to take the brunt of an energy blast directed against the Caped Crusader. Batman carries J'Onn to his personal jet while Superman provides cover. A squadron of invader spacecraft give chase to the Batwing while the Martian sends out a telepathic distress signal. Green Lantern John Stewart is the first to respond, while Superman can't seem to evade alien fire and vanishes. Hawkgirl joins the fray almost immediately after, while the Alien Atlas recovers and takes flight. He draws fire, then turns intangible to allow one craft to down another, repeating as able. Batman loses his plane and his usefulness for the time being, while Hawkgirl needs a rescue from the late arriving Amazing Amazon. Wonder Woman's a bit of a "rookie" on the show, however, and needs a rescue herself from Lantern. Flash brings up the rear.
The battle won, J'Onn J'Onzz explains the origins of himself and Earth's latest invaders. Since Superman was so successful at disarming our planet's nuclear reserves, only the heroes gathered by the Martian have any hope of defeating the evil doers. From the aliens' beachhead in Metropolis, a base is constructed and terraforming begins to allow the nocturnal invaders to blot out the sun's rays. Flash wonders why they couldn't just use the same nerve gas that defeated the invaders on Mars. "The gas can only be made from a rare Martian plant. I brought a sample with me, but it was destroyed when I was captured."
The heroes divide into teams, the Martian joining Wonder Woman and Batman in infiltrating the Metropolis base. J'Onn pauses, acknowledging through his telepathy "Superman and Hawkgirl are down. They have failed." J'Onn leads his team to the heart of the invaders' headquarters, alerting Batman that they can shut the operation down by stealing the "ion matrix crystal." J'Onn uses his flight, intangibility, and strength to do just that, but is felled by a trooper's laser fire. While Wonder Woman carries him to safety, Batman recovers the ball, though he was separated from his fellows in the process. Batman appears to perish, a sight J'Onn confirms as fact...