Darkonda, an evil monster/scientist, joins the Dark Fortress crew, much to Ecliptor's cagrin. He unleashes an alien insect on the Rangers, known as the Barillian Bug. Carlos is bitten by it, and once Andros realizes just what it is, a deadly insect that once attacked his homeworld. Cassie also ends up stung, this time by Carlos himself, who mutates into a Barillian monster! It'll take a trip back to the abandoned planet of KO-35, to search for any remaining antitode to save the Rangers from a permanent change of species.