ThunderCats Season 1 Episode 57

Willa is captured by the Mutants, and Tygra and Nayda, Willa’s sister, must go to Castle Plun-darr to save her. Tygra uses the invisibilty power of his whip on both of them, so they could sneak into the castle and rescue Willa, but having 2 people at a time takes too much power from the whip, so they are stuck being invisible. Lion-o uses the Sword to make them visible once again and Willa is rescued.