S1·E56 The Invisibles

A chunk of magnalite lands in the Lost Lake on Quasar. Before long, the Herculoids are up against enemies they can't see. Zandor discovers that the invisible adversaries are Zelos. The team goes to see King Zel, who reveals that his son Eezo and other exiled Zelos may be the culprits. The Herculoids head for the Valley Of The Lost Lake. There, Gloop, Gleep, Tundro, and Dorno's arm turn invisible. Space Ghost, who has been tracking the magnalite, arrives, and sends the fragment back into space. Once this is done, the invisibles become visible again. The Herculoids capture Eezo and his followers, and return them to King Zel.