1x12 - Sabrina's Secret Life

While fighting over the leading role in the school play next to Harvey, Cassandra puts something in Sabrina's brew during cauldron class that makes it an invisiblity potion. The girls use the potion to get back at each other until it cause only parts of them to become invisible, and they have to go to the middle of nowhere in the Netherworld or fade away to nothingness.