Plot: Two years ago, the Joker spared Charlie Collins's life in exchange for a favor. Now, on the eve of a banquet in honor of Commissioner Gordon, the Joker calls the favor in.
The Joker is especially psychotic in this episode. He obsessively tracks Charlie to another state, noting that Charlie "owes me a favor!" as if the Joker might forget that. He's fixed his hatred on Gordon, moving from previous targets Mayor Hill (who cameos) and Batman himself. Even the nature of favor is insane. Charlie just has to open a door, which either of the Joker's two thugs who were also in the room could have done. All of his actions move him from being an extreme personality to one that has actually become unfathomable. You just cannot understand why he would torment someone for two years, only to kill him off-handedly.
Of course, skipping to the end, we learn that the Joker is of course, just a bully. A lot of his power comes from the fear he induces in his victims beforehand, making people believe that only Batman can stop him. But one good punch to the solar plexus and little "I might be crazy" routine and the Joker changes his tune right quick. Of the four Joker episodes so far, this is by far his most satisfying defeat. Not only does he have to beg Batman to save him again, but instead of tripping or getting one good kick the chest, he's actually out witted and out joked, and by a "nobody" at that.
Charlie makes a pretty great "everyman," with pudgy features, bald head with a pathetic attempt at a combover, and his own theme music pulled out of a 50s sitcom. He's a loser, but he's not a bad guy. He is motivated by his family's safety and he's surprisingly resourceful, basically inventing the Batsignal in a world that lacks it. Ed Begley Jr. voices him with such a put-upon tone, like he was defeated long before he crossed paths with a supervillain.
But, of course, the really big story is that this is the first episode with Harley Quinn, who is easily the most popular character created by the show. You can see her development from Be a Clown, because she is exactly what the Joker was trying to turn Jordan Hill into, a mini-version of himself who is also his biggest fan. Harley literally cheers almost everything the Joker says, and finds his plans brilliant and his jokes hilarious. We know she's not just any thug, because she has her own costume. We don't know her origin, a couple lines to Batman implies that she's new on the scene and possibly a beauty school drop out, but that doesn't get at just how dark her relationship with the Joker actually is.