“Skin of Evil” remains one of the most shocking and controversial episodes in the long-running saga of Star Trek. More often than not, the different eras of the franchise have taken a while to find their space legs, and that’s particularly true of the first live-action sequel to The Original Series. Star Trek: The Next Generation may have presented a more enlightened Federation of the 24th century than its 1960s predecessor, but its first year was notoriously troubled.