Lt. Mira Romaine (Jan Shutan), whom Scotty has fallen for, takes an important role when she is somehow connected with the bizarre non-corporeal entities called the Zetarians, who were responsible for deaths on a Federation outpost. Romaine suddenly begins having visions of the future, including one in which Scotty is dead.
While "Lights" is not an incredibly insulting episode, it is surprisingly devoid of substance. Really, there's not much that happens in the story. The lights appear, cause strange visions and take control of Romaine's body, and are destroyed when Kirk puts Romaine in a high-pressure chamber to expel them from her body.
Much of the episode consists of lackluster scenes where the crew attempts to determine the nature of the Zetarians and their hold over Romaine, and a few scenes analyzing Romaine herself as a crew member having trouble adjusting her attitudes to her new assignment. There's simply not much here worthy of mention for good or ill (though the inability for an understanding to be reached between humanity and the Zetarians is perhaps a telling sign of the decline of the series' idealism). Overall, a tolerable but fairly pointless episode.