For the first time this season, David has a possible cure: using a pharmaceutical drug to suppress the right side of his brain (the irrational side, for those who missed high school biology). It backfires, creating an enzyme which periodically turns him into evil Banner, and makes the Hulk more aggressive. This further embroils David in the problems of the family he's staying with: Daddy just got back from 10 months at sea, and 18-year-old Laurie doesn't take kindly to his prolonged absences.
The three IMDb reviews preceding this one all say the same thing: This is a very good episode, but it could have been a lot better. That echoes my own take on it.
The chief problem is that evil Banner is so lacking in subtlety that he's often ridiculous rather than menacing. Surprisingly, this is less due to Nicholas Corea's script than to some very hammy acting from Bill Bixby. While Bixby does deliver a few moments with the villainous brilliance he showed in "Broken Image", overall he tries too hard, even forcing out several cringe-inducing "Mwah-ha-ha-ha"s. He turns in a much better performance as normal Banner, particularly in the sequences where he struggles to keep his evil side from taking control.
Evil Banner does more than just wreck a little havoc: He sets his lustful eyes on Laurie. Laurie's amorous response to his attentions, and his lust for a girl half his age, give this episode a genuinely disturbing note. Bill Lucking plays the brutish overbearing father, and his performance in the role is significantly better than when he did it in "The Antowuk Horror", mainly because he has better material to work with. The final scene with him and Laurie is genuinely touching. And really, it's an interesting twist to see David inflaming the problems of the supporting cast instead of solving them.