Somewhere in the Milky Way, the alien gladiator Despero is aboard his ship and has his robotic major domo, L-Ron, review the files of the Justice League. L-Ron admits that most of the powerful League members are on Rimbor, but assures his master that Earth is a breeding ground for super-powered individuals and that they will find an adequate challenge for him.
At the UN, Secretary Tseng Dangun and the Reach's ambassador meet with Captain Atom at the General Assembly. The ambassador asks Captain Atom if the League might use its power to enforce diplomatic relations and Captain Atom invites them to discuss the matter in private. As they leave, commentator Gordon Godfrey talks about the private meetings and accuses the League of hiding their true agenda from the common people of the planet.
At Star Labs in Taos, Black Canary interviews the Reach abductees that Young Justice rescued from the alien headquarters. Virgil Hawkins is the next abductee that she speaks with, and he explains that the Reach scientists shocked him in an attempt to get him to react. He tells Black Canary that some of the abductees disappeared for good when they failed whatever test that the aliens were administering. As he leaves, Black Canary notices that Virgil has unconsciously magnetized the paper clips that he was toying with.
Nightwing takes some of the team members to Bludhaven where they are using a warehouse as a temporary dormitory while they set up base in the Hall of Justice. Nightwing shows Miss Martian her headquarters and apologizes for the lack of privacy. Malcolm says that he has other arrangements and leaves for the Hall of Justice. Nightwing asks Conner and Miss Martian to go with him and bring back some of their salvaged personal effects from Mount Justice so that they can make the dormitory more comfortable.
When Malcolm arrives at the Hall of Justice, he suggests that he move in with Bumblebee. She hesitantly refuses, saying that she isn't allowed to have roommates and is spending all of her time studying. Before Malcolm can arrive, L-Ron flies down and announces himself, calling the heroes outside. He surrounds the Hall with a force field and tells them that they are now his prisoners. Despero teleports in and L-Ron introduces his master. The alien gladiator attacks Captain Marvel, smashing him through the wall, and L-Ron announces that the cage match has begun. When Zatanna tries to cast a spell, Despero uses his third eye to telepathically sedate her rather than allow magic in a gladiatorial battle. While Captain Marvel attacks Despero, Malcolm, Bumblebee, Conner, and Miss Martian try to get out without success. Conner goes to help Captain Marvel while Bumblebee tries to wake Zatanna up without success.
Captain Atom meets privately with Tseng and the Reach Ambassador. Captain Atom accuses the Reach of hiding on Earth before announcing themselves, and the Ambassador claims that his people stayed in hiding because of the League's reputation as galactic criminals. When he claims that the League had its young cover operative team target his ship, Captain Atom reminds him that he abducted their people. The Ambassador says that it was the Kroloteans and Tseng asks Captain Atom if he has any evidence. The Ambassador is also glad to hear from any witnesses, but he points out that young impressionable teenagers would not be acceptable. Before he can respond, Captain Atom receives an alert about the crisis at the Hall of Justice.
Black Lightning and the military try to penetrate the force field without success. Inside, Despero continues to bead down Conner and Captain Marvel as Malcolm and Miss Martian arrive. Miss Martian telekinetically attacks Despero but refuses to brain blast him. Despero throws Captain Marvel through an exhibit of retired heroes and the hero returns to the attack, invoking his magic lightning and letting it hit Despero as it transforms Captain Marvel back into Billy Batson. The attack fails and Despero telepathically sedates Billy rather than fight a child. L-Ron helpfully reminds him that Conner is wearing the Superman symbol and could prove a worthy opponent.
After Impulse finishes his session with Black Canary, Blue Beetle talks to him privately and Impulse assures him that he didn't tell the heroine about how Blue Beetle will start a Reach apocalypse in the future. He assures Blue Beetle that since he doesn't know what turns Blue Beetle evil, he doesn't want to risk creating it by saying the wrong thing to the wrong person. The scarab, satisfied, telepathically tells Jaime to kill Impulse to keep their secret but the boy refuses.
Bumblebee and Miss Martian try to help Conner, while the non-powered Malcolm notices a discarded Guardian costume from the exhibit. Despero prepares to kill Conner and Miss Martian, still traumatized by accidentally hurting Aqualad with her mental abilities, refuses to launch a telepathic blast. Bumblebee attacks Despero but her stings are useless and he telepathically sedates her as well. As Despero prepares to kill her, Mal dons the Guardian costume, steps out, and tells Despero that he is Earth's true champion.
When Black Canary talks to Blue Beetle, he tells her about the scarab and how the Reach couldn't reboot it without killing him. The heroine suggests that they could use the scarab as a weapon against its creators.
L-Ron is unable to identify Mal from the Krolotean database and admits that they could be in error. Miss Martian telepathically links with Mal, who tells her to come up with a plan while he keeps the aliens busy. She plays along, bowing to him as Earth's champion, and then gets Conner and Bumblebee out of the room. Mal puts on an act, pretending to summon the powers of the Earth, and dates Despero to attack. When the alien charges at him, Mal dodges out of the way using Black Canary's training.
When Conner wakes up, he tells her that she needs to focus. She still refuses to use her mental powers and he tells her to come up with another plan if she can't... or won't.
Despero finally grabs Mal, realizing he has no powers and is just a distraction, and Conner returns to the fight. The alien subdues both of them, while Miss Martian manages to wake Zatanna up. The sorceress then casts a spell through Miss Martian's telepathic link to Mal, turning Despero's back on himself. A furious L-Ron goes into battle mode and prepares to destroy them, but an awakened Billy jumps down on him, summons his magic lightning, transforms back into Captain Marvel, and smashes the robot. It self-destructs and Captain Marvel shields his teammates as the Hall of Justice collapses on top of them.
Later, the heroes are unable to get through the force field and the Reach ambassador arrives. He uses a device to shut down the force field and Captain Atom reluctantly thanks him through gritted teeth. The ambassador casually mentions the League's Watchtower headquarters in orbit and then apologizes, claiming that he didn't know it was a secret. Godfrey is watching the newscast and immediately announces to his audience that the League is monitoring everyone on the planet from their secret satellite.
Back at the warehouse, Nightwing and the others watch the broadcast. When Nightwing wonders why mal returned, he simply says that he's staying there and doesn't mention his discussion with Bumblebee. Miss Martian isn't there and Nightwing tells his teammates that she's made other sleeping arrangements. In Chicago, Miss Martian uses her shapeshifting powers to disguise herself as her uncle and moves into the apartment that Martian Manhunter uses in his civilian ID. Lagoon Boy tries to call her but she ignores it and answers the door. It's Conner, who says that they need to talk.
Later, Captain Atom and Nightwing meet with Black Canary in Taos. She explains that after her interviews, she's confirmed that all of the abductees either had innate superpowers or the potential to develop them. She now believes that the Reach is trying to weaponize the metagene that humans possess, using it to their own advantage. Blue Beetle comes in, ignoring Impulse, and tells them the truth about how he is supposed to turn evil and aid the Reach's takeover of Earth. Retracting his armor, Jaime asks them to remove the scarab no matter what it takes.
Despero is a long-time Justice League villain in the comics who first appeared in Justice League of America #1 (October-November 1960). Originally a fairly scrawny villain with a much bigger head-fin (set parallel to the shoulders), he later mutated into a gigantic form similar to what he see here. Originally possessing telepathic powers and advanced technology (and an obsession with chess - he used the Leaguers as game pieces in at least two different stories), he later gained superior strength sufficient to let him simultaneously withstand the assault of the mightiest members of the Justice League and Justice Society (in the Virtue & Vice graphic novel).