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.
The satellite dish sabotaged earlier is finally restored, only to detect a massive alien vessel headed for Earth. Television reporter Snapper Carr breaks the news that this craft is five times the size of the originals. Riots break out, as observed by Wonder Woman, who wonders if her mother was right about Man's World being filled with nothing but savages. J'Onn J'Onzz was more forgiving. "Do not judge them too harshly. They act out of fear." As if in response, a pair of bikers turn up to help children trapped under debris.
Flash and GL answer the Martian's summons, and the trio launch an initiative to rescue Superman and Hawkgirl. J'Onn J'Onzz assures that they yet live, but time is short, as he also senses the Imperium was coming. "The Who?" queried Flash. "The supreme intelligence that controls these invaders. We have met before." Flash questioned Green Lantern, "Can we trust this space case?" John replied, "What choice do we have?"
Returning to the base, the Martian assumes the form of an invading trooper, leading a pair of the real thing aside until he can send them into shock (via the variable density of his arms passing through their chests.) "Is it just me, or does he creep you out too?"
The group was eventually led into a trap, and gassed unconscious. J'Onn J'Onzz is awoken by Superman, who condemns the heroes for risking their freedom for his own. "Would you have done less for me?" The collected heroes are soon joined by Senator J. Allen Carter, or rather the invader who has impersonated the astronaut since the Mars expedition, and used Superman to disarm Earth. Next to arrive is the Imperium, an elliptical purple blob with tentacles. "J'Onn J'Onzz? It's been a long time." Troopers use a sort of extraterrestrial cattle prod to shock the Martian back to his natural form. "Much better. You have defied us for centuries."
"And I will never bow before you or any of your kind."
That being the case, the Imperium wishes to eliminate the last Martian himself, penetrating his flesh with its tentacles and absorbing the hero into its body. "After all these years, you have finally lost." Except, the Martian was hiding something... the Dark Knight! "I mentally shielded him so he couldn't be detected." Batman uses technology to reverse the ion charge that powers the base, allowing sunlight to reach the Earth again. It seems the invaders are not just nocturnal, but catastrophically vulnerable to ultraviolet rays. How the senator managed to avoid it all those years, I can't say, but he perishes this day. The last Martian burst free from the Imperium's gelatinous mass and drug* it into the tormenting sun by its tentacles. "You live underground and shun the light. Why? Does it burn your pale, putrid skin?" It does indeed, which is why it electrocuted the manhunter and made its escape. However, its exit vessel is destroyed by Hawkgirl and Wonder Woman, as the invaders are wiped out.
Fearing what might happen the next time such a threat rears its head, Batman has a satellite base constructed to house the new super-team suggested by Superman, a Justice League. Everyone present agrees to join, though Wonder Woman wonders, "Wait-- J'Onn's not here." Superman found him observing space on an elevated platform.
"J'Onn... you alright?"
"My family and loved ones are long gone. I am the last of my kind... Now, Mars is dead, and I am alone in the universe."
"J'Onn, We can never replace the family you've lost, but we'd be honored if you could learn to call Earth your home."
The aliens smiled at one another, and a new alliance is formed...
*drug: dialect past of drag per Webster's. I hate the word "dragged."