As alien invaders who were accidentally awakened on Mars begin to take over the Earth,Superman andBatman rescueMartian Manhunter, who telepathically summonsWonder Woman,Hawkgirl,the Flash, andGreen Lantern to help stop the invasion. Superman gathers everyone at the newly built Watchtower, funded by Batman, and asks them to be part of a team, which he dubs the Justice League.

Green Lantern surrenders himself to roboticManhunters to stand trial for the destruction of a planet. While Flash acts as Green Lantern's attorney, the rest of the League discovers thatKanjar Ro helped the Manhunters frame Green Lantern in a bid to steal the Green Lantern power battery.

A nuclear submarine is attacked in the Atlantic, forcing the League to come face-to-face withAquaman and his Atlantean army. Aquaman follows Superman's advice to go toMetropolis and take his problems up with the various governments of the world. When a mercenary,Deadshot, attacks Aquaman, the League steps in. Aquaman discovers thatOrm, his brother, was behind the attack. Orm takes Aquaman and his infant son and places them on a cliff over a vein of molten lava, leaving them to die. To save his son, Aquaman sacrifices his own hand.

Exposed as a criminal, sentenced to prison, and terminally ill due to long-termKryptonite exposure,Lex Luthor escapes and assembles asupervillain team, theInjustice Gang, to take on the Justice League. The gang consists ofCheetah,Star Sapphire,Shade,Copperhead,Ultra-Humanite, andSolomon Grundy.Joker also becomes part of the team, on his own initiative and due to his expertise on Batman. Even with their combined power, however, the gang proves no match for Batman's guile.

Having spent nearly eight months with the League and feeling she owes her motherHippolyta an explanation for her leavingThemyscira abruptly to fight in the Imperium invasion, Wonder Woman returns to her home only to encounter the sorcererFelix Faust, who has petrified theAmazons and will not return them to normal unless she gathers artifacts for him. Faust uses the artifacts to releaseHades fromTartarus. The League succeeds in exiling Hades and his minions, but Wonder Woman is exiled from Themyscira for allowing the men of the Justice League to help.

Superman andMartian Manhunter are abducted by alien slave traders and transported toWarworld, whose rulerMongul pits aliens against each other in gladiatorial combat. Hawkgirl and Green Lantern set out to rescue them. During this time, Superman meetsDraaga, who is chosen to become Warworld's leader after Mongul is defeated.

The Flash and Green Lantern uncover a plot byGorilla Grodd to destroyGorilla City, a hidden city in Africa inhabited by intelligent gorillas. Grodd has a device that allows him to control the minds of others and uses stolen gorilla technology to conquer Central City and put its inhabitants under his control. Locked inside the city, Flash and Green Lantern work withSolovar, Gorilla City's chief of security, to stop Grodd before he carries out his revenge.

Arogue Amazonian named Aresia, who was born in the mortal world and raised on Themyscira, forms another incarnation of the Injustice Gang with Star Sapphire, Shade, Copperhead, Solomon Grundy, and a new villain named Tsukuri. She sets out to steal a ruby to use it as a virus to kill the world's male population, whom she holds responsible for the death of her family in a war. When the male members of the League are infected by the virus, putting them out of commission, Wonder Woman and Hawkgirl must work with Hippolyta to stop Aresia.

Flash, Green Lantern, Martian Manhunter, and Hawkgirl are accidentally transported to a parallel world and discover theJustice Guild of America, who are comic book characters in the Justice League's world. As they traverse this world and help the Justice Guild of America fight the Injustice Guild, they soon discover that not everything is what it seems since the end of their comic series.

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Note: The episode is dedicated toGardner Fox, a Golden and Silver Age comic writer who co-created theJustice Society of America and the Justice League.^[4] Originally, the showrunners wanted to use the JSA, but DC Comics publisherPaul Levitz felt that the story was inappropriate, so they altered the names and designs of the team.

Jason Blood and his alter-egoEtrigan the Demon seek the assistance of the Justice League in preventing thePhilosopher's Stone from falling into the hands of his ancient enemy, the sorceressMorgaine le Fey.

Green Lantern's old friend Rex Mason, now working for a shady industrialist, suffers an "accident" arranged by his jealous employerSimon Stagg and is transformed into the superheroMetamorpho.

The Justice League returns from a mission in space to find the world transformed due toVandal Savage altering history, allowing him and theNazis to winWorld War II. Finding the machine Savage used to send his past self information, the League uses it to travel back in time to stop him and fight Nazis alongside WWII-era heroesEasy Company, theBlackhawks, andSteve Trevor.

The League is tricked into defendingDarkseid's homeworld ofApokolips againstBrainiac, despite Superman's skepticism and his anger attheir previous encounter. The team solicits the aid of theNew Gods, includingOrion,Lightray, andHighfather. In the ensuing battle, Brainiac and Darkseid are presumed dead, though Superman believes otherwise.

Lex Luthor manipulates a powerful android namedAmazo with the ability to copy any superpower, posing as a father figure to it. Meanwhile, Martian Manhunter questions the nature of humans and struggles with the burden of hearing thousands of their minds in his head at once, after performing a psychic sweep of Metropolis in a failed attempt to locate Luthor. After gaining Manhunter's telepathy, Amazo discovers that Luthor was using him and leaves Earth to discover his purpose.

Small time crook John Dee volunteers for an experimental treatment while in prison. While the authorities are distracted by a mass prison break led byVolcana, Luminus,Copperhead,Solomon Grundy andFirefly, John Dee overdoses on the treatment and becomes the dream-controllingDoctor Destiny. Destiny takes telepathic control of most of the League in their sleep and puts them in nightmares. This leaves only Batman and Martian Manhunter left. Martian Manhunter tries to force them awake, while Batman, resisting sleep, goes to confront Destiny.

Wonder Woman befriends Audrey, the princess ofKasnia, who is reluctant to end her wild ways to get married. Unfortunately, her fiancé is Vandal Savage, and he already has plans for both the throne and the Justice League. Wonder Woman and Batman must stop Savage, while Flash, Green Lantern, and Martian Manhunter take out Savage's new Rail Gun satellite.

Kilowog crashes to Earth, seeking Green Lantern's help to rescue other members of theGreen Lantern Corps from the psychic conquerorDespero. He also explains that Green Lantern's former mentor and lover,Katma Tui, is one of the captives. After facing Despero and losing, Green Lantern has to retrain himself to use his power ring to help the League end Despero's reign. Flash, Hawkgirl and Martian Manhunter accompany Kilowog to Despero's planet to help Green Lantern stop him.

In an alternate reality, the Justice League launches an assault on the White House, where Superman kills President Luthor in retaliation for the Flash's execution by Luthor. Two years later, the League, now called theJustice Lords, have become tyrannical rulers. After Justice Lord Batman creates a method of dimensional travel, the Lords travel to the main universe and replace the League, attempting to make this Earth like their own. After finding their way back to their world, the Justice League ends up having to turn to their world's version of Lex Luthor for help.

An ancient lunar crystal called the Black Heart is discovered. Unbeknownst to the members of the Justice League, it harborsan evil snake spirit with the ability to possess its bearer. The crystal's power contaminates almost all of the League, except for the Flash, who must save the League from a weapon that will destroy both the Sun and the Earth.

Doctor Fate and Aquaman rescue Solomon Grundy, intent on using him to help battle an ancient evil. Superman, Hawkgirl and Wonder Woman are conflicted about this goal.

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Note: The pairing of these heroes is an homage to theMarvel Comics superhero teamthe Defenders. The plot is based uponH. P. Lovecraft'sCthulhu Mythos.^[6]

While the Justice League are quarreling about the value of mutual trust and teamwork, Gorilla Grodd organizes aSecret Society composed ofGiganta,Killer Frost,Sinestro,Parasite, Shade, andClayface. The Society captures most of the League, but Martian Manhunter frees the others, and the Society is defeated in front of a crowded stadium.

A band of supervillains (composed ofMetallo,Kalibak,Livewire,Weather Wizard, andToyman) team up to get revenge on Superman. When they attack Metropolis, Superman sacrifices himself to save Batman and Wonder Woman from Toyman's weapon and is seemingly vaporized. The League attempts to cope with the loss of Superman by defending Metropolis in his absence, save for Batman, who does not believe Superman is dead and begins investigating what happened. Unbeknownst to the League, Superman is transported 30,000 years in the future, where Vandal Savage has devastated Earth with a gravitational weapon. Superman works with Savage to return to the present and stop Savage's past self, altering the timeline and erasing the bad future.

With his Gwynplaine Entertainment Company (a reference toThe Man Who Laughs) the Joker takes over TV stations inLas Vegas, announcing that he has placed a series of bombs that will destroy theLas Vegas Strip if they are not stopped by the Justice League. With the whole world watching, the League must first get past the Joker's own super-team, theRoyal Flush Gang. However, the threat imposed by the Joker is not as simple as it seems, and Batman must track down his location to stop him.

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Note: This episode is notable as a 'hidden crossover' withTeen Titans, as the Royal Flush Gang is voiced by the five principal actors from that series. The gang's designs are also based on theTeen Titans voice actors.^[7]

After saving two worlds, the members of the Justice League decide to take a break to celebrate the holidays. The Flash spends his Christmas Eve with the children of an orphanage, who ask that he find them a special animated toy duck. The popular toy is broken by the Ultra-Humanite, who eventually comes around and helps The Flash. Green Lantern and Hawkgirl show each other how they spend the holidays. Green Lantern engages in a snowball fight with Hawkgirl. Hawkgirl responds by starting an interplanetary barroom brawl. Batman spends Christmas on Watchtower duty. Meanwhile, Superman invites Martian Manhunter over toSmallville for Christmas Eve withhis adoptive parents. Although Martian Manhunter is initially uncomfortable, he eventually learns to enjoy Christmas.

After Earth is attacked by aGordanian battleship, the League is aided by soldiers from Hawkgirl's home planet of Thanagar. TheThanagarians offer to build a shield to defend Earth against the Gordanians, but Batman discovers that this is a ruse: the Thanagarians intend to build a portal to attack the Gordanian homeworld, which would destroy Earth. Hawkgirl finds herself torn between her allegiance to Thanagar and her commitment to the Justice League, as well as between her betrothal to her commander Hro Talak and her love for Green Lantern.

When theWatchtower has a sudden power drain, the Justice League recruitsStatic to jump start their station before it reenters the atmosphere. Unbeknownst to either Static orGear, the power drain also releasesBrainiac from his confinement. Static and Gear manage to bring Brainiac down, but he escapes and possesses Gear and the Justice League.

Static faces off against his idol Green Lantern (actuallySinestro in disguise) when he causes chaos all over town.

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Note: Although this is a crossover withJustice League, only Green Lantern appears.