After a century of silence, the Romulans, a race with whom humans had a war long ago, appear to be on the move again—apparently testing their new cloaking technology—and the Enterprise may be the only ship to stop them from starting another conflict.
The episode is in the tradition of submarine thrillers like Run Silent, Run Deep (or The Hunt for Red October as a more current comparison), but like "The Corbomite Maneuver," it has a tendency to get too wound up in its tactical moments and special effects, which can be confusing and sometimes go on for too long. And, I'm sorry, the set design on the Romulan ship is just not convincing; unlike the Enterprise, it feels too much like a cheap set and little more. And what's with those corny helmets the Romulan officers wear? There are, of course, many strengths here; the episode takes some stabs at relevant issues, especially when the crew discovers the Romulans may be related to Vulcans and a bridge officer's bigoted aggression (aimed here at Spock) emerges.
The Cold War allegory was certainly timely, and the idea of each captains' respect for their counterpart makes for a nice theme, but I wish it had been taken a bit farther. Mark Lenard is good as a Romulan commander who begins to doubt his military's usefulness, but his thoughtfulness isn't utilized nearly as much as it could've been.
The episode's unevenness is its drawback, and it's too bad; there's the potential for greatness here that goes unrealized. Each little theme would've been more effective if given more focus. "Balance" needed more balance.