When the family misbehaves at the power plant picnic, Homer grows concerned that his family is too dysfunctional and proceeds to put everyone in to family therapy with shocking results (pun intended).
As mentioned, this first season episode is a tad jarring for modern Simpsons fans to watch. Almost the entire family (with the exception of Bart, who was really the only one with a defined personality by this point) is acting out of character. Homer's concern about his family's behavior seems more like something expected from Marge, especially seeing as he's willing to pawn the family's TV set to pay for the therapy sessions. Marge getting drunk at the picnic is more like something that Homer would do. And Lisa seems more like a female Bart and is almost as bratty as he is.
With this in mind, the therapy session is clearly the major highlight of the episode. It's also the first episode where Bart utters the immortal line, "Don't have a cow", Mr. Burns first says, "Release the hounds", and it's the first appearance of Itchy and Scratchy, so it features some significance in Simpsons history.