At the Metropolitan Museum of Art, the police investigate the theft of the Sword of Beowulf. The curator reassures them that there is nothing to worry about, as the sword's power cannot be used unless the wielder is pure of heart and speaks the incantation, "Abannan Afol Beowulf". Detective Daniels is not impressed, and remarks that the thief must be long gone. However, Harm shows himself and reveals that he overheard the incantation. He speaks the incantation, unlocking the sword's power, and is easily able to take out the police. Frightened, the curator stammers that only one pure of heart could wield the sword, but Harm replies that the legend did not specify the wielder had to be pure good. He kills the curator and leaves.
Wally, Megan and Conner get into their costumes, a werewolf, a zombie bride, and a mummy respectively, to go to the Happy Harbor Halloween Dance. Captain Marvel discovers he can't go to the dance with them, much to his disappointment, and leaves via the Zeta-Tube. Artemis and Zatanna arrive, dressed as a vampire and a witch respectively. Seeing Megan work on Conner's costume, Zatanna asked Artemis how long they had been a couple. Artemis replies she doesn't know, and is visibly upset. Seeing this, Zatanna casts a spell to change her and Artemis out of their Halloween costumes and into their superhero costumes, and suggests they skip the dance and go on patrol.
Megan, Wally and Conner arrive at the Halloween party. Much to Conner's chagrin, nearly all the other partygoers are dressed as Justice League members. They meet up with their friends Mal, Karen, Wendy and Marvin, who is focused on reports about a Martian invasion.
At Mount Justice, Batman holds a meeting with Red Tornado, Red Arrow, Robin and Aqualad to discuss the possibility of a mole within the Team. Aqualad no longer believes there is a mole because all the Team members helped during the fight against the Injustice League, which he reasons is something a mole wouldn't do.
Zatanna tries to get Artemis to talk about Superboy and Miss Martian, but Artemis prefers to express her frustration through violence. They set off and foil several crimes. Harm spots them and decides to hunt them for sport and to practice for when he goes up against their mentors Zatara and Green Arrow. With the sword, he is superior to both young heroines, forcing them to retreat. Zatanna and Artemis are helped by a mysterious girl who can only mutter the word "secret", who leads them away from Harm.
Red Arrow doesn't trust Artemis, because he knows she isn't Green Arrow's niece, which Aqualad did not know but Robin had already figured out. Red Arrow also doesn't trust Miss Martian, as he heard from Black Canary that Martian Manhunter has only known her for five months. Batman is still unsure of what happened to Superboy at Cadmus, and suggests Superboy may be the mole and not know it.
At the Halloween party, the power goes out. A voice over the PA system claiming to be Homeland Security warns the students to stay inside, leading the students to believe Marvin's stories about the Martian invasion. Conner, however, uses his infrared vision to check out the gymnasium, and sees Marvin at the PA system.
The mysterious girl, whom Zatanna and Artemis nickname Secret, leads them to a ruined house. Harm catches up to them, and is furious that they've found his house. He defeats them with the sword and captures them.
Marvin steps outside the gymnasium. To get back at him, Conner, Megan and Wally sneak outside with him and use their powers to stage a fake Martian invasion in which Wally and Conner appear to be killed. Shocked with fright, Marvin runs back into the gymnasium and warns everyone that the Martians killed Conner and Wally, but the two reveal themselves to be in the gym, perfectly OK. Marvin sheepishly confesses his prank.
Harm interrogates Artemis and Zatanna to discover how they found his house. Secret helps Zatana free herself, and she rescues Artemis by distracting Harm with an illusion spell. Secret leads them to the backyard. The duo learns Secret's real name is Greta Hayes, and that she was Harm's sister but is now a ghost, having been killed by Harm years ago.
Harm arrives and when confronted with the story, he boasts that he was proud of it. Greta was the only thing he loved, and for his heart to be pure, he had to get rid of her. Upon seeing the ghost of his sister, he shrugs it off as another illusion spell, but Zatanna points out that she did not speak any spell. Harm vehemently denies any feelings of guilt, insisting that he's not sorry. But it's proven to be untrue as Secret walks towards him and takes away the purity of his heart. The sword now no longer answers to him, and Artemis is able to make short work of him. As the police arrive, Zatanna promises Greta they'll give her a proper burial. Seeing Artemis's troubled face, Zatanna urges Artemis to tell her the truth, because secrets obviously don't stay buried. But Artemis refuses, saying that she has no secrets. Artemis then shows Zatanna that right across the street is the magic store Abel's House of Secrets, with a broken neon sign leaving only the word "Secret" highlighted. She supposes that it was the last thing Greta saw before her death.
Artemis keeps secrets from the rest of the Team and is unwilling to reveal them. Batman says that she must be permitted to have a secret identity.
Batman, Red Tornado, Red Arrow, Aqualad, and Robin have a secret meeting to discuss the issue of the mole within the Team.
The identity of the mole, or if there even is one, is a secret.
Harm's secret is that he killed his little sister, Greta Hayes, in order to become pure.
The last thing that Greta Hayes saw before she died was the word "Secret". As a ghost, this was the only thing she could say. Because of this, Artemis and Zatanna referred to her as Secret until they learned her true name.
Batman, Red Arrow, Robin and Aqualad discuss the revelation of a mole by Sportsmaster, which happened in "Targets".
They also discuss the fight against the Injustice League from "Revelation" and that they all believe the Injustice League to be the masterminds of the villain activity, still unaware of the Light.
Artemis's crush on Superboy is brought up once again, which was hinted in "Infiltrator" and "Denial".
Kid Flash is still flirting with Miss Martian and oblivious to the fact that she and Superboy are a couple.
Zatanna mentions a move she had practiced after fighting with the Reds in "Humanity".
Red Arrow points out that Artemis isn't really Green Arrow's niece, which Aqualad was not aware of. This was first revealed in "Infiltrator".
Robin then shows that he was aware of who Artemis is, and begins to reveal the truth of who she's really related to before being cut off by Batman. The fact that Robin knows more about Artemis than the others do was first suggested in "Homefront".
Harm is voiced by Ben Diskin who had previously voiced Venom in The Spectacular Spider-Man, another Greg Weisman production. Both characters exhibit idiosyncratic speech patterns: Venom spoke in the first-person plural to refer to himself ("we"), while Harm refers to himself in the third-person singular and uses the impersonal pronoun to refer to others ("it", "they" and a pluralized "its"). Moreover, both are psychotic and both have bound and gagged a character voiced by Lacey Chabert.
One of Zatanna's spells ("Erif nrub, nordluac elbbub") translates into "Fire burn, cauldron bubble", which is a line from William Shakespeare Macbeth (Act Four, Scene One). This line was also used in the recreated scene of the play in the episode "City of Stone" Part Three, from series producer Greg Weisman's Gargoyles.
Abel's House of Secrets is a reference to the House of Secrets, a mansion that played a large role in mystical and magical DC stories since the 70s, including House of Secrets itself. Abel is the biblical Abel, son of Adam and Eve, who served as the host. His brother Cain lives in the House of Mystery and was the host in the sister comic of that name.
Wally's costume is that of "Teen Wolf", the eponymous character of the 1985 fantasy comedy film starring Michael J. Fox.
Wally asks Megan, "So, you going as my favorite Martian?", making a subtle reference to the 1963 sitcom My Favorite Martian, featuring a telepathic Martian living on Earth.
On the wall concealing the Zeta-Tube to Manhattan there's a mural with the title "The Silver Blade", which can be a nod to the 1987 supernatural fantasy comic book series Silverblade, written by Cary Bates.
When Megan comes out as a Martian to trick Marvin she looks like Marvin the Martian from Looney Tunes and her roar is identical to that of Godzilla.