Sabrina's cauldron blows up and splatters Cassandra with purplescent slime... and when she tries to clean it off, every place it touches her, it turns her invisible (for a short time). Sabrina's comment that she always knew Cassandra was only "half-there" doesn't help things, either. But when Cassandra makes the best of it -- using the invisibility slime again and again to cheat in school, spy on Sabrina, get cozy with Harvey, etc. - Sabrina decides to fight fire with fire and slimes herself invisible. But when the spell slime starts to not wear off as planned, Sabrina and Cassandra must call a truce and work together to find a cure before they're never able to come back!