The Joker infects fish with laughing gas. After not being allowed to copyright his Joker Fish, he targets innocent people.
- Disproportionate Retribution: The Joker introduces his "smile" toxin into the fish supply of Gotham Harbor, hoping to trademark the red-lipped, grinning ichthyoids and sell them in supermarkets. When told that he cannot trademark fish he retaliates by carrying out an elaborate scheme to kill everyone in the Gotham City patent office until he gets his way.
- The End... Or Is It?: Played with. The Joker fell into the ocean very close to a hungry shark. Commissioner Gordon asks Batman a bit later if he thinks the Joker's really dead. Batman says he wishes so, but that he has his doubts. The camera closes in on the Joker card that Harley has thrown into the ocean in mourning and then it's eaten by the same shark, managing to make the viewer certain both that the Joker survived and that he did so only by the skin of his teeth (so to speak) at the same time.
- Force Feeding: A comedic example. Joker is doing a commercial for his new brand of Joker Fish, and tries to get Harley to eat one to show how good they are. Harley sheepishly protests that she's got a problem with fish, but Mr. J just shoves it in her mouth. She manages to grunt out "Yummy-yum-yum" before running offscreen to hurl.
- Mermaid Problem: The Joker seems to be flirting with Harley Quinn by asking her to "be [his] little mermaid." He then puts a giant fake fish head over her head, to which Harley responds, "You're really sick, you know that, boss?" Funny this got past the censors.
- Shark Pool
- Shout-Out: Joker pulls a wrench out of a box with the words "Binford Tools" written on it.
- Spared by the Adaptation: The episode loosely adapts a 2-part comic story of the same name. In it, Joker infects two random victims with a lethal dose of his joker venom. In the TAS version, since broadcasting standards wouldn't allow the writers to kill someone in a children's cartoon, instead Batman is successfully able to administer an antidote to the venom after both get infected. Doesn't stop the scene from being any less freaky, though.
- Squee: Harley, when Joker says he's gonna make her his little mermaid.
- Window Pain: It's not a rock, but one of the poisoned Joker Fish that comes through the window of a predicted victim's home.