In New York City, Lt. Daniels investigates the theft of the Sword of Beowolf from a museum. The curator tells him the incantation that goes with the sword, and Daniels figures that the thief is gone. A figure drops from the ceiling with the sword, introduces himself as Harm, and recites the incantation. The sword powers up and he easily disposes of the police and then the curator, and steps out onto the streets of New York.
At Mount Justice, Miss Martian and Superboy dress for their Halloween dance at the local high school. They've invited Kid Flash along, and he thinks Miss Martian wants him along for a date, unaware of her relationship with Superboy. Captain Marvel assumes that she invited him as well but she didn't. Disappointed, he goes trick-or-treating as Zatanna and Artemis arrive. Zatanna immediately picks up on the fact that Superboy and Miss Martian are a couple, and Artemis is surprised and shocked. She storms off and Zatanna goes after her, suggesting they take a girl's night out.
Later at the high school, Kid Flash, Superboy, and Miss Martian arrive for the party, and most of the students are dressed as superheroes. Miss Martian introduces her classmates, and Marvin checks the Internet and gets reports of a Martian invasion. The other students get similar texts.
At Mount Justice, Red Tornado, Batman, and Red Archer meet with Aqualad and Robin, who insist that there is no mole on the team. Red Archer points out that Artemis isn't Green Arrow's niece, much to Aqualad's surprise, and Batman stops Robin from telling his teammate the truth. The League members still suspect Superboy, unsure what Cadmus may have done when cloning him, or Miss Martian, who has only recently met her uncle. Aqualad insists that he knows and trusts them implicitly.
Artemis and Zatanna see the commotion at the museum, and Artemis wants something to beat up. She continues on, unaware that Harm is watching her and Zatanna from the rooftops. The two teens stop several street crimes as they drive through the night, Artemis eager to get some action rather than think about the relationship between her two teammates. Harm finally attacks them, blowing up their motorcycles, while a robed girl watches the encounter. Artemis and Zatanna fight back, but the sword deflects arrows and magic alike. He realizes that they studied with Green Arrow and Zatara and prepares to fight them to learn how to defeat their mentors. Zatanna tries to summon the sword to her, and Harm throws it at her. As she ducks, Artemis fires but Harm still catches the arrows and throws them back.
Zatanna creates a circle of fire to hold Harm back and the two teens run away, and see the robed girl gesture to them. They follow her into an alleyway and discover that she's already made her way to the roof. When they get there, the girl introduces herself as Secret. When Harm jumps onto the rooftop, Zatanna tries to embed him in ice but he breaks free while Secret gets to a neighboring rooftop. Zatanna and Artemis cross to her with Harm in hot pursuit.
At the dance, the power flickers and reports of invading Martians continue coming in. Homeland Security advises everyone to stay inside but insists the invasion isn't confirmed. Superboy uses his abilities to scan the crowd and spots Marvin at the microphone, making the announcements and sending the viral warnings.
Harm knocks Zatanna aside and Artemis dares him to fight without the sword. He accepts the challenge and sheathes the sword, and then drives it into the rooftop. Artemis attacks him but Harm notes that she's battling her inner demons and can't focus. When Zatanna tries to free it, the backlash stuns her and Harm knocks down the distracted Artemis. Secret waves to them from another building and Artemis and Zatanna run to her, only to discover that she's lead them to the end of the rooftops near a magic shop. Harm arrives, demanding to know how they've come to his home in the building below them, across from the magic shop. He uses the sword to shock them and then stands over them, Secret standing next to him.
When the freshman students to try leave, Marvin orders them back in. However, Superboy, Kid Flash, and Artemis stage their own fake Martian attack on him. Miss Martian shapechanges into a "Martian," knocking Superboy out, and Marvin runs screaming back into the school. The teen heroes go inside and watch as Marvin rants about invaders, saying that he pulled a prank earlier. They then reveal that they pranked him and he sheepishly admits he pulled a prank.
Artemis wakes up and finds herself tied to a chair. Harm comes in and shows her a camera image of Zatanna, tied up in another room, and he warns that he'll harm her if they don't tell him how they found his home. Artemis points out that his partner lured them there, but Harm says that he works alone. Meanwhile, Secret approaches Zatanna and removes her gag, allowing her to free herself with an incantation. She asks what Secret is up to, but then realizes that she's in Secret's bedroom and finds a photo of the girl and Harm, together as brother and sister.
Harm spots Zatanna on the screen and realizes that he's free. He runs in to attack her only to discover that she's distracted him with a conjured image. Zatanna frees Artemis and assures her friend that Secret is on their side. Harm arrives to attack them but they run out the back and Harm realizes that Secret has left the gas running. It ignites, blowing up the building and Harm.
Outside, Zatanna and Artemis emerge into the back yard and find a grave marking Greta Hayes, Beloved Sister. Secret emerges from the grave and they realize that she's a ghost. She shows them Harm's dagger and they realize that he killed her. Harm emerges from the building and explains that Greta was the only thing he ever loved, so he killed her to remove the contamination. Secret appears and Harm assumes it's another of Zatanna's spells, but the heroine says that his secret is out. He insists that he's not sorry, but Secret walks to him, reaches into his chest, and removes a glowing ball of energy. He tries to attack her but his sword goes through the ghost and then it tears itself away from him since he's no longer pure. Artemis knocks him out, pointing out that now he's the unfocused one, and Zatanna binds him with a spell. They tell Greta that she'll receive a proper burial and Secret descends back into her grave. Artemis says that she can't believe someone would do that to his own sister, and starts to talk about her own family. Zatanna suggests that she get her own secrets off of her chest, but Artemis angrily insists that she has no secrets. As the police arrive, the heroines see the last thing that Greta ever saw: the sign for the magic store, with only the word "Secret" lit up.
William "Billy" Hayes first appeared in Young Justice #4 (January 1999), and was created Peter David and Todd Nauck. An orphan, Billy is adopted by the Hayes family, but sacrifices his sister Greta to gain powers from the demon Buzz. Determined to become the world's greatest murderer, Billy takes the name Harm and goes up against Young Justice. He is killed by his adopted father but returns from the dead during Day of Judgment and seeks revenge against both Young Justice and his adopted family. Harm has super-human strength and speed, and the ability to possess the minds and bodies of others. This is his first animated appearance.
Greta Hayes first appeared in Young Justice: The Secret and was created by Todd Dezago and Todd Nauck. Her adoptive brother Billy killed her to gain power from Hell, but Greta was trapped between dimensions, becoming an ethereal figure trapped between life and death. She was captured by the Department of Extranormal Operations and rescued by Robin, Impulse, and Superboy. She served with Young Justice for several years, but was eventually corrupted by Darkseid. Darkseid eventually stripped her of her abilities, leaving her an ordinary girl. As Secret, Greta can transform into a mist-like substance, teleport, alter her own ghostly form to any shape she desires, and take souls to their just reward. This is her first animated appearance.