At a grocery store a robbery is thwarted by Green Arrow. Green Lantern shows up to help him and brings him to the satellite via teleporter. Superman informs the assembled heroes they will now be working all together and J'onn will coordinate specialized teams and send them into a field. There's a strong radiaoactive signal spotted in China and J'onn assembles a team of Green Lantern, Supergirl, and Captain Atom. Green Arrow insists on going with them so he can drop them off.
They arrive and local militia open fire on them. They are greeted by General Kwan who tells them to leave. Green Arrow goes off on his own to track the radiation and comes to the aid of a local soldier. As he argues with General Kwan, they see a huge flaming behemoth who opens fire on the army. Green Arrow tries to distract it but his explosive arrows are useless. The others show up as GL rescues him just in time. Captain Atom tries to drain its energies but there's too much and he's knocked out.
Supergirl engages the creature while GL conducts rescue operations. The behemoth attacks GL, badly injuring him, then escaping. The three heroes consult with the local hazmat teams and manage to find out the creature's weakness despite its classified nature as a creation designed to "protect" China from outsiders. They have to shove a carbon rod into the behemoth's rods and Captain Atom volunteers for the duty. Supergirl distracts the creature while Captain Atom makes his run despite the intense heat. He fails, and his containment suit is breached, forcing him to get clear just before as he blows up. Supergirl goes down and Green Arrow grabs a containment rod and tries to hit the slot in the reactor. He constructs a makeshift arrow and makes the shot, with Supergirl administering the coup de grace.
Back at the satellite they manage to restore Captain Atom's energies. Green Arrow and Batman have a chat and Green Arrow decides to stay both to keep the League honest and meet Black Canary.
Green Arrow is a Golden Age hero created by Mort Weisinger and originally drawn by George Papp (in More Fun Comics #73, November 1941). As one site put it, "create" may be too strong a word - Green Arrow was a virtual clone of Batman - wealthy millionnaire Oliver Queen, red-costumed sidekick (Speedy, later Arsenal, who shows up over on Teen Titans), drove around in a Arrowcar and hung out in the Arrowcave. He basically had everything Batman did except for a distinct personality and interesting opponents - he did join the Seven Soldiers of Victory, however, along with Speedy. He progressed to the Silver Age, and joined the Justice League (issue #4, April/May 1961) where he remained pretty generic and bland, pretty much indistinguishable personality-wise from the other members under Gardner Fox. When he lost his own comic, the League writers decided to "kick him up" - they stripped him of his fortune, changed his costume, had him grow a beard, and he became a new "liberal" type of 70's hero. He became outspoken and rather obnoxious and the first League member to display a distinctive personality. He became romantically involved with Black Canary (thus the bit at the end of this episode), and later became the costar in Green Lantern's comic and left and rejoined the League several times. The Golden Age Green Arrow was incorporated into the Earth-Two setting as were the Seven Soldiers (which met the Silver Age Justice League, although the two Green Arrows never met that I can recall). The Earth-2 Green Arrow died during the events of Crisis and was eventually retconned so he never existed anyway.
The Earth-1 (main) Green Arrow eventually "died" and was replaced in the League by his illegitimate son Connor Hawke who took on the role of Green Arrow, but Oliver got resurrected and got a new series initially written by filmed movie director Kevin Smith.
Supergirl is a Kryptonian who first appeared (in the WB animated universe, in the episode "Little Girl Lost"
. She survived by being put into stasis by her parents. Her cylinder survived on a chunk of Argos, Krypton's twin planet, which Superman eventually found - he then revived her. She took on the identity of Supergirl and appeared in various WB animated series, occasionally teaming up with Batgirl. Prior to this, we had last seen her in the last episode of the Superman animated series, "Legacy"
, helping him against Darkseid. It's not clear to what degree the public believes she may be related to Superman in some manner. In the comics, her origin is substantially different. Pre-Crisis she's Superman's cousin - post-Crisis she's had at least three different ones, including a new one in the current Superman/Batman comic.
Captain Atom was originally a character created by Steve Ditko at Charlton Comics who first appeared in Space Adventures 33 (March 1960). He had a pretty undistinguished career, and eventually DC got the rights to the Charlton stable of characters. He first appeared (briefly) in Crisis where it was revealed that the Charlton "universe" was another of the multiple Earths. That Earth was destroyed and/or merged with all the others (don't ask) and Captain Atom became an established hero on Earth and appeared in Captain Atom #1. His origin here is different - in the comics he is encased in an alien metal as part of a government experiment. Here his nature is similar to that of Wildfire of the Legion of Super-Heroes - a being of sheer energy given form by a containment suit. Captain Atom is the character upon which Dr. Manhattan of the Watchman graphic novel is based upon.
Brimstone originally appeared in the Legends mini-series, where he was a high-tech semi-sentient creation constructed by Darkseid's minions and sent to Earth to undermine its heroes. It was taken down by the newly-formed 80's Suicide Squad in a manner not dissimilar to what we see here - a crack marksman hits Brimstone in a critical location with a special missile and deactivate/destroys him. He's since shown up occasionally when Darkseid has nothing better to do. His origin here, needless to say, is substantially changed.
The assembled heroes are a rather odd mixture. There are several representatives of the original 1940's Justice Society (Johnny Thunder, Wildcat), a team which didn't seem to exist in the Justice League's universe (since the JSA was active in World War II in the comics, but the soldiers in Savage Time seemed unaware of the idea of superheroes). Several heroes (Atom-Smasher), Sand, Mr. Terrific II, Stars and S.T.R.I.P.E., Hourman, Dr. Mid-Nite II) are members of the modern-day incarnation of the JSA. Several heroes have appeared in past episodes (Dr. Fate, Aquaman, Metamorpho, the Demon). Steel III (John Henry Irons) appeared in two episodes of Superman: The Animated Series. Many of the heroes have been members of one incarnation or another of the Justice League in the comics - the Atom, Dr. Light 2, Red Tornado, Atom-Smasher (as Nuklon) and Obsidian (also tied to the JSA), Elongated Man, Fire, Ice, Aztek, Aquaman, Dr. Fate, Metamorpho, Huntress, Steels II & III, Booster Gold, Vixen, Zatanna, Orion, Black Canary), Gypsy, and Vibe. The Creeper has been loosely affiliated with the League (and previously appeared in the Batman Gotham Knights episode Beware the Creeper). There seems to be a fascination with the Charlton heroes: Captain Atom, Nightshade, and the Question (although Blue Beetle, an actual member who is fairly popular, is absent as another company holds the rights to the character at the moment so DC can't authorize Warner to use him). And many of the heroes are independents who have never been part of the team: Vigilante, Starman (Prince Gavan), Blue Devil, Shining Knight, B'wana Beast, and Nemesis. And finally, there's a shot of the Crimson Avenger, who is now recognised as the first of DC's Golden Age super-heroes.