It's 911 in the morning and 1-900 in the evening, the phone-sex EMS dispatcher, when Sick, Sad World returns.

Daria's science class gets assigned a project: make a maze, and train a rat to run through it. Miss Barch, the science teacher, pairs Daria with Kevin, and Brittany with Upchuck.
Daria distracts Kevin with their cable TV, and he watches the Pigskin Channel while Daria works on the project. Quinn tries to get Kevin to date her, but he doesn't seem to notice.
Brittany becomes convinced that Daria and Quinn are trying to steal Kevin from her, and a rumor goes around school about Kevin dating Daria and Quinn. Upchuck does their project, while using Brittany as his personal slave after blackmailing her with a picture of her with the quarterback of the rival football team.
Brittany steals Daria's rat to get back at her, and then gives it to her little brother.
Daria figures out who stole it, and gets it back by telling Brittany that Kevin will have to help her with a make up project. It's been traumatized by Brittany's brother, and won't move when placed in the maze, but Daria gets an A on the project anyway because of her spiel on conditioning.
Daria makes several pointed comments about conditioning to both Miss Barch and Helen, but neither of them notice.

Jane:  Maybe you could get a wind-up toy to distract him.
Daria:  His wind-up toy's working with Upchuck

"I can't.  I signed a confidentiality paper.  The other scientists would be mad." - Daria ("The Lab Brat")

Daria:  Kevin, what are you doing here?  Did you get lost?
Kevin:  No.  I mean... sorta.

Brittany:  Kevin, the new Whitney Houston movie is out, and I want you to take me, tonight.
Kevin:  Can't, babe.  Gotta work with Daria.  You know, science.
Daria:  That's all right, Dr. Pasteur.  You can take the night off.
Kevin:  Daria, that's a chick movie.
Jane:  Psst, Kevin.  Brittany's a chick.

Miss Barch:  Excellent job, Daria.  You get an A.
Kevin:  Alright!
Miss Barch:  Not you, you man.  You get a D.
Kevin:  Alright!

Wraith's Ramblings:
We start exploring the minor characters some in this episode, mostly Kevin and Brittany but a nice (but even more minor) part for Charles. We're also introduced more to Ms. Barch. All four of them are pretty basic stereotypes, so there's not all that much introduction necessary.