From an episode where I didn't have much to say to one where I may have too much. So let's get right to it.
In Pretty Poison, Bruce Wayne's pal, Harvey Dent, has met a new lady love, Pamela Isley. But when the D.A. is felled by poison, Bruce discovers this blushing flower hides deadly thorns as the villain Poison Ivy.
However, the Animated Series economic storytelling hinders the what could be a typical noir mystery, "Who poisoned the District Attorney?". Even if you didn't know Batman has an established villain named Poison Ivy, the title and the fact that only one new character is introduced in the whole episode point pretty loudly to Pamela Isley's guilt right away. We know she did it, the only real question is why.
Which is how we get to the character of Poison Ivy. Like the Scarecrow, Ivy is motivated by revenge, though she is slightly better justified. After all, she seeks vengeance on behalf of the environment, and there's some irony that the only plant that could cure Harvey is the rose bush he almost eradicated by building his prison. That's as far as justification goes, however. In the end, she's still killing a man for almost destroying a rose, one she herself is willing to harm if it helps her achieve her goals.
And that's the overt stuff. It's be hard to argue that the vertical maw of the Venus Batman Trap is not a representation of vagina dentata (what a wonderful phrase). And the animators slipped in another gag as the greenhouse is located at "69 Greene St."
This episode is also important in establishing Harvey Dent and Bruce Wayne's relationship. The flashback reveals that they have been friends for at least five years by this point (and answers my nagging question of how long has Bruce been Batman). Harvey comments on Bruce's wealth to Pamela, and in fact it's Bruce Wayne's credit card paying for the meal, suggesting a strong class difference between the two. On the other hand, Harvey Dent has the respect and political power that Bruce lacks. If Bruce is still worried that being Batman is a disgraceful way of honoring his parents, perhaps he looks at Harvey as the man he should have been. They trust each other enough to listen to each other's advice, but not completely. Harvey's line, "there's nothing we don't know about each other," is doubly ironic. Obviously, he doesn't know Bruce is Batman, but we'll later learn that Harvey is keeping a big secret from Bruce as well.
Also, Ivy's thrown into Stonegate Prison, the source of her hatred of Harvey Dent, and not Arkham Asylum, despite being obviously crazy. Presumably the judge felt the irony was too irresistible.
Batman once again demonstrates his lack of trust of anyone else, stealing the blood sample from the doctor treating his friend Harvey Dent. You'd think the doctor would need that.