Trini's famous scientist uncle Howard drops by the Youth Center, with a new invisibility formula. Rita spots it, and plans to use it on her new monster, the camouflaged creature known as the Dark Warrior. Billy gets sick of Bulk & Skull picking on him, and decides to reenroll in Jason's karate class. But as before, he doesn't fare well in it. Uncle Howard, not only a scientist but a master martial artist, offers to give him some training at the park. There, Dark Warrior kidnaps him, and orders him to give up the invisibility formula. He refuses, and gets strapped to a bomb capable of leveling the city. Billy saves the day, and manages to get his yellow belt with Uncle Howard's help.