They Came from Outer Space (2) - S2.E4

M.O.M. reveals Billy's hidden life as Ganthar, destroyer of worlds, before he abandoned his old name, form, and way of life and joined the Center. She sends the gang to trade the captured aliens for Billy, unaware that once the aliens reunite, they will be able to call in reinforcements and destroy the Earth! The only one who can stop them is Billy, but to do that he will have to become Ganthar again— and he might not want to change back.