On a spaceship somewhere in the Milky Way, the alien gladiator Despero reviews the files of the Justice League. His assistant robot, L-Ron, points out that most of the League is on Rimbor. However, it shows that there are others on Earth, which acts as a breeding ground for super-powered opponents.
At the UN, Secretary Tseng and the Reach's ambassador invite Captain Atom to meet with them. The hero is glad to play along but the Ambassador drops several hints about how the Justice League could use their powers to enforce diplomacy. Captain Atom suggests that they talk about it privately and the three of them leave. Commentator Gordon Godfrey seizes on the private meeting and suggests that the Justice League is hiding their true agenda from the people.
At Star Labs in Taos, the teenagers rescued from the Reach are waiting for their examinations to confirm that they're okay. Black Canary escorts Virgil Hawkins in next and tries to find out what happened as well as provide counseling. All Virgil knows is that the Reach wanted to break him by administering increasingly stronger electrical shocks. He tells Black Canary that some of the kids disappeared and asks when he can go home, and Black Canary asks him to wait outside. As he goes, Black Canary notices that Virgil has unconsciously electrified some paper clips.
In Bludhaven, Nightwing takes the team to the warehouse that they will be using as a temporary resident hall because Mount Justice was destroyed. Meanwhile, the team will use the basement of the Hall of Justice as their actual headquarters. Nightwing escorts Miss Martian to her shared quarters and apologizes for the lack of privacy, while Malcolm decides to leave and go to the Hall of Justice. Conner and Miss Martian go with him to bring in whatever they were able to salvage from Mount Justice.
At the Hall of Justice, Malcolm meets with Bumblebee and suggests that he stay with her. She refuses, pointing out that her place is small and she isn't allowed visitors. Before Malcolm can respond, L-Ron arrives outside and calls out to the heroes. He informs them that they are his prisoners and surrounds the Hall of Justice with a force barrier. L-Ron then introduces his master... Despero, who teleports in. The alien smashes Captain Marvel into the hall and L-Ron declares that the match has begun.
Zatanna prepares to use a spell on Despero but the alien shuts her down with the power of his third eye, considering magic cheating. L-Ron explains that Despero is a gladiator seeking a challenge to prove his superiority. Captain Marvel flies back onto the attack while Malcolm and the others confirm that they are entirely sealed off from the outside world. Conner and Bumblebee fly to help Captain Marvel and Conner fights Despero while Bumblebee discovers that Zatanna is entranced. When that fails, she's unable to blast a hole in the force field.
Captain Atom meets with the Reach ambassador and Tseng, and points out that the Reach hid on Earth for a while before making their presence known. The ambassador notes that the League has a reputation as criminals and targeted his ship with their young covert ops team. When Captain Atom points out that the Reach abducted their people, the ambassador points out that was the Kroloteans were the ones doing that. Tseng asks for evidence and the ambassador is glad to hear from any witnesses who aren't teenage fans and teammates. Before he can respond, Captain Atom receives a signal about a crisis at the Hall of Justice.
General Eiling and his soldiers try to blast through the force field around the Hall without success. Meanwhile, Despero continues to smash down Captain Marvel and Conner as Malcolm and Miss Martian arrives. Miss Martian telekinetically throws a statue at Despero, who easily smashes it apart and throws the wreckage back. When Malcolm suggests that Miss Martian brain blast Despero, she adamantly refuses. Despero throws Captain Marvel through cases of retired heroes and then smashes him repeatedly. Captain Marvel responds by speaking his magic world, summoning the magic lightning to strike Despero and transform himself back into Billy Batson. Despero is unharmed and telepathically sedates Billy rather than fight a child. L-Ron seeks out a new opponent and spots Conner, wearing Superman's symbol.
After Impulse finishes talking with Black Canary, Blue Beetle draws him off to the side. His friend assures him that he didn't say anything about Blue Beetle's causing the Reach apocalypse in the future. Impulse explains that he doesn't know what makes Blue Beetle turn evil and doesn't want to risk provoking it by saying anything to the League. The scarab mentally tells Blue Beetle to kill Impulse to keep their secret, but Blue Beetle refuses.
Despero and Conner fight and Bumblebee and Miss Martian try to help their teammate. A powerless Mal is soon left behind, but he notices the discarded costume of the Guardian and gets an idea. Meanwhile, Despero finally knocks Conner out and prepares to kill him. Bumblebee tells Miss Martian to make a mental attack, but she remembers her unwitting attack on Aqualad and insists that she can't. Bumblebee fires, getting Despero's attention, but he telepathically knocks her out. As Despero prepares to crush her, Mal arrives in the Guardian costume and declares himself Earth's champion.
Black Canary talks with Blue Beetle next and points out that the Reach created his scarab. Blue Beetle explains that the Reach created it but couldn't control it without killing him, and Black Canary points out that the scarab could be their greatest weapon against the alien invaders.
L-Ron points out that Mal isn't listed in the Krolotean databases but they could be in error. Meanwhile, Mal mentally tells Miss Martian to come up with a plan while he keeps Despero busy. She bows to Mal and flies the unconscious Conner and Bumblebee out, and L-Ron figures that Mal is a worthy opponent. Mal continues to play up his assumed role, "summoning" the powers of mankind and daring Despero to attack. He then ducks out of the way, taunting and avoiding Despero.
Conner tells Miss Martian that she needs to focus and tells her to come up with a plan if she won't use her mental attacks.
Despero finally grabs Mal and slams him to the floor, and Conner pulls him down through the floor. However, the alien quickly realizes that Mal has no powers and that it's another distraction. As he captures Mal and Conner, Miss Martian manages to penetrate Despero's telepathic sedation. Zatanna casts a spell through Mal via the telepathic link to turn Despero's mental powers back on himself, rendering him comatose. Conner knocks the alien out and L-Ron goes into battle mode to avenge his master. Billy drops down through the ceiling, transforms back into Captain Marvel, and uses his lightning to blast L-Ron apart. The robot's head self-destructs, destroying the building, and Captain Marvel manages to shelter his teammates.
Later, the heroes are still trying to bring down the force field when the Reach ambassador arrives. He uses a small device to shut it down and Captain Atom reluctantly thanks him. The ambassador mentions the League Watchtower on national television, and then apologizes, claiming he didn't know its existence was a secret. Godfrey is quick to seize on the news and continue decrying the League.
Back at the warehouse, the team watches Godfrey's broadcast. Mal says that he's staying without mentioning Bumblebee's rejection, and Nightwing informs them that Miss Martian has made other sleeping arrangements. In Chicago, Miss Martian moves in to her uncle's apartment, using her shapeshifting powers to disguise herself as him. She receives a call from Lagoon Boy but ignores it, and Conner comes to talk to her.
Captain Atom, Nightwing, and Black Canary meet at Star Labs and Black Canary says that all of the Reach abductees were either known superheroes or those with the potential to become superheroes. She believes that the Reach is attempting to cultivate the human metagene and weaponize it. Blue Beetle comes in and prepares to tell them the truth, but Impulse speeds in and advises him to stop. The scarab agrees, but Blue Beetle tells the heroes that he's destined to bring about the Reach apocalypse. Mentally retracting his armor, Jaime asks them to get the scarab off of him whatever it takes.