Harlequinade

"Harlequinade" is the seventh episode of the second season of Batman: The Animated Series. It originally aired on May 23, 1994. The Joker steals an atomic bomb, and it is up to Batman and Robin to find and stop him. Since there is no time to spare, the duo enlist the help of Harley Quinn to track Joker down. Throughout the episode, it is briefly discussed how Harley met Joker.

Plot
The Joker steals an enormous bomb from a mob auction, plotting to "make Gotham one big dent". Mayor Hill refuses Commissioner Gordon's request to evacuate the city, saying that they don't have enough proof that the bomb is still in Gotham to risk mass panic. Not knowing who else to turn to, Batman and Gordon plan to offer Harley Quinn, currently incarcerated in Arkham Asylum, a full pardon in return for helping them.

Harley cheerfully (and suspiciously) accepts the offer. She first leads Batman to Joker's most recent hideout. Joker is not there, but Batman is appalled to find that Joker has the entire city under video surveillance. While he is looking at the screens, Harley changes into her usual harlequin outfit—seemingly oblivious when Joker's hyena's Bud & Lou attack Batman.

Batman holds them off with difficulty, and Harley calls her "babies" to her. Finding the hideout a dead end, Harley and Batman then crash a mob gambling party, run by Boxy Bennett, where Batman is captured. As Robin sneaks in to aid Batman, Harley sings a dark yet humorous number, "Say that We're Sweethearts Again" to distract the gun-toting party goers. After they escape, Batman gets impatient and demands to know where Joker is. Harley laughs and mocks him for not having figured it out already: Joker is holding Mayor Hill prisoner at his mansion, using Hill to distract the cops.

After handcuffing Harley in the Batmobile, over her protests, Batman and Robin sneak onto the mansion's grounds, finding the bomb on the lawn and Joker frolicking in Hill's swimming pool. Hill asks that Joker just make his ransom demand, the city will gladly pay it. Joker says he's not planning to ransom the city—instead, he's just going to explode the bomb and take care of the cops and Batman in one fell swoop. He and his goons already have an old-fashioned biplane set up on the grounds to escape the explosion.

Batman and Robin appear, and fighting ensues. Harley, escaping her cuffs, appears and ties up both of the Caped Crusaders. "Deal's off, B-Man. Nobody said anything about hurting Mr. J". Joker is somewhat surprised to see Harley there, but cheerfully invites her to board the plane with him. He starts the bomb's timer: 10 minutes.

Robin points out that Joker didn't leave enough time to swing by Arkham and save Harley. Joker laughs this off, but Harley also realizes that there isn't enough time to save their fellow inmates. Since the clock is ticking, Joker tries to override her objections, but she panics when Batman mentions their hyenas. "I'll buy you a goldfish!" Joker yells, exasperated, "let's go!" Harley resists, and he flies off without her.

Harley frees Batman and Robin just as Joker circles around, strafing the ground with the plane's machine gun and dropping bombs. Batman manages to evade them, and Robin successfully defuses the bomb. Now totally crazy, Joker sends his plane on a suicide dive toward the bomb, firing the machine gun at it, saying it's going off, one way or another. But Harley fires a pinpoint shot with a spring-loaded Joker head, hitting Joker and knocking him into a daze. He loses control of the gun and shoots his own fuselage, and the plane crashes into the Mayor's mansion.

As Joker stumbles out of the wreckage, Harley levels the machine gun at him. Batman yells for her to stop, but they are too far away to prevent her. Joker scoffs, saying she doesn't have the guts to shoot him. She pulls the trigger, Joker flinches—and a flag that reads "RAT TAT TAT" pops out of the barrel. Batman and Robin sigh in relief. For a moment, Harley looks afraid of Joker's reaction, then he laughs and says "Baby, you're the greatest!" They embrace each other, their nonsensical romance rekindled.