Don't Have a Cow is a 2003 Halloween special of the Disney Channel sitcom That's so Raven. In the main plot, Raven and Chelsea try to use a wishing spell to become popular at a party, but a misstep causes the two girls to slowly turn into cows. They try to stop the spell at the party before the lively teens become livestock. In the side-plot, Cory tries trick-or-treating without his dad.
Eddie reveals that he was invited to Alana's party by her friends to Raven and Chelsea. Neither of the girls were not invited due to Raven's rivalry with Alana. Later they go to the Chill Grill where they find out that Raven's father, Victor, allowed Alana to have the party at the Grill (which Raven thinks Alana did to spite her). They start eating their meals, but Chelsea freaks when she discovers that Victor accidentally switched her and Raven's burgers, and is ashamed by her actions since she essentially just ate a cow as a vegetarian.
At Raven's house, Chelsea starts wearing a button with a cow's face on it featuring a no sign (likely encouraging people not to eat meat), still mortified by her actions. Raven suggests they look into her Aunt Viv's chest for old magical stuff for fun, and they discover a spell book containing a spell that can grant wishes. They conjure up the ingredients in a cauldron and wish to go to Alana's party, which hypnotizes Alana into calling them and inviting them. They then use it to wish they were the most popular girls at the party and win the costume contest, and in their excitement don't notice that Chelsea's button fell off into the cauldron, which starts affecting their wish as well as themselves.
While preparing their costumes, Chelsea notices that Raven's ears were long and pointy like a cow's, mistaking it for a costume part. They start freaking out when it turns out it's Raven's actual ears and are further frightened to find that Chelsea has a long, furry tail on her back. To make matters worse, Eddie arrives at the house (dressed as a wizard) to go to the party with them. They force Eddie out of the house with the book (as it goes with Eddie's costume) to avoid him seeing their new body parts, and Raven starts mooing in a low pitched voice similar to how a cow sounds. They find the button in the cauldron and realize that they are slowly transforming into cattle because Chelsea's button made it an accidental wish. With Eddie having the spell book, they decide to go to Alana's party to get it back and and reverse the spell, figuring that since it's Halloween, it won't matter if they look like two farm animals.
At the party, the spell continues changing Raven and Chelsea into cows by morphing their body parts (such as giving them snouts and turning their hands into hooves) and affecting their behavior (Chelsea drinks out of the punch bowl with her face and Raven starts chewing cud). They find Eddie, but Chelsea ends up eating the page with the instructions to reverse the curse, dooming their only chance. Alana's friends announce the winners of the contest and are surprised to find Alana in second. Alana angrily runs up and discovers Raven and Chelsea won. At this point, the two are completely transformed into Holstein cows and decide to "milk" the moment. It's then revealed that the whole episode was a vision, and Raven stops Chelsea from eating the burger, preventing the whole thing from ever happening.
In the side plot, Cory (who's dressed as a weightlifter) decides that he wants to trick or treat with his friend William (who's dressed as a remote), disappointing Victor (who's Dr. Frankenstein). Tanya (dressed as Mrs. Frankenstein) suggests that she and Victor go trick or treating on their own. Cory and William end up missing out due to Will's costume setting off alarms in the neighborhood while his parents get bagfuls of candy. Feeling guilty, Cory admits what happened to Victor, and they all go trick or treating to make up for what Cory missed.