As Halloween approaches, the Joker escapes from Arkham Reform School and Barbara is excited to be able to go trick-or-treating with Harleen Quinzel in Gotham City. Diana persuades the other girls to go as well so she can secretly keep watch on Harleen, whom she alone knows to be Harley Quinn (see "The Warrior and the Jester"). During the outing, Barbara gets a call from Batman alerting her to the Joker's plans to make this Halloween the scariest one ever; at the same time, Harleen receives a call from the Joker asking for her help. The girls suit up and split into pairs to investigate three targets mentioned in Batman's call; Wonder Woman tries in vain to persuade Batgirl to break off her friendship with Harleen, without revealing the secret. The targets prove to be traps that lead to the release of the Gentleman Ghost, Solomon Grundy, and She-Bat, and Batgirl discovers that the Joker faked Batman's call in order to set his plan in motion.
As the Joker and Harley lure the three villains to them, Wonder Woman tells Batgirl that Harleen is Harley. Batgirl refuses to believe her at first, but later accepts the truth after thinking about the events of "The Warrior and The Jester." She and Wonder Woman reconcile, and all six heroes confront the villains as they begin causing mayhem throughout the city. The Joker and Harley capture Batgirl and Wonder Woman, and the Joker reveals his plan to destroy Gotham with a bomb that Harley is to detonate, much to Harley's hidden horror. While the other four heroes subdue their opponents, Batgirl unmasks herself as Barbara to persuade Harley not to help him. Harley pushes the detonator anyway and flees with the Joker, and Batgirl throws a Batarang in an attempt to defuse the bomb. It does not explode, but she finds that she missed her throw and that Harley had secretly disarmed it, suggesting that there may yet be hope for her to turn from evil.
- Kimberly Brooks as Karen Beecher/Bumblebee
- Grey Delisle Griffin as Diana Prince/Wonder Woman, Victoria Craddock
- Phil LaMarr as Security Guard
- Tara Strong as Barbara Gordon/Batgirl, Harleen Quinzel/Harley Quinn, Francine Langstrom/She-Bat
- Nicole Sullivan as Kara Danvers/Supergirl
- Fred Tatasciore as Solomon Grundy, Gentleman Ghost, Mayor of Gotham
- Myrna Velasco as Jessica Cruz/Green Lantern
- Kari Wahlgren as Zee Zatara/Zatanna
- Jeremiah Watkins as The Joker
- This is the first episode that is centered on Halloween.
- As of this episode, Babs and Harleen now know each other’s secret identity.
- Unlike most episodes, this 2-parter has a longer credits scene.
- Fred Tatasciore reprises his role as Solomon Grundy from the G1 counterpart.
- This episode marks the final appearance of Nicole Sullivan voicing Supergirl before leaving the show. Therefore, voice directors will find a replacement as they make a new episode. 
- The ringtone in Barbara's phone is the iconic 1989 Batman theme composed by Danny Elfman.
- In Batgirl's explanation of Harley secretly switching the Plunger Detonator off with her foot, the flashback shows that both her shoe and stocking were both red rather then it being a red shoe and black stocking.
- Diana acts like she never said "Great Hera!" but says it all the time.
|v • e Generation 2 animated media|