This episode marks the first appearance of the Romulans on Star Trek.

Mark Lenard (Romulan Commander) would later play Ambassador Sarek, Spock's father, in "Journey to Babel".

This is the final appearance of Grace Lee Whitney (Yeoman Janice Rand) on the series.

The basic plot of this episode is that of a film titled The Enemy Below in which a destroyer commander and a U-boat commander fence across the Atlantic.

This was the first episode re-mastered for the 40th anniversary of Trek in 2006. Special effects were updated and CGI of the Enterprise replaced the original model work, and color balance was generally tuned up for high-definition.

Kirk: Since the days of wooden vessels, shipmasters have had one happy privilege--that of uniting two people in the bonds of matrimony.

Spock: Earth believes the Romulans to be warlike, cruel, treacherous, and only the Romulans know what they think of Earth

Stiles: I doubt they'll identify themselves. They're painted like a giant bird of prey.
Kirk: Is history your specialty?
Stiles: Family history. A Captain Stiles was in the space service, and several officers--all lost in that war, sir.
Kirk: Their war, Mr. Stiles. Not yours. Don't forget it.

Uhura: Cryptography is working on it, sir.
Stiles: Give it to Spock.
Kirk: I didn't quite get that, Mr. Stiles.
Stiles: Nothing, sir.
Kirk: Repeat it.
Stiles: I was suggesting that Mr. Spock could probably translate it for you, sir.
Kirk: I assume you're complimenting Spock's abilities.
Stiles: I'm not sure, sir.

Kirk: Leave any bigotry in your quarters; there's no room for it on the bridge.

Romulan Commander: Danger and I are old companions.
Centurion: We've seen a hundred campaigns together, and still I do not understand you.
Romulan Commander: I think you do. No need to tell you what will happen. The Earthman will follow; he must. And when he attacks we will destroy him. Our gift to the homeland: another war.

Romulan Commander: Another war...must it always be so? How many comrades have we lost in this way?... Obedience. Duty. Death, and more death...

Romulan Commander: First study the enemy. Seek weakness.

Spock: If Romulans are an offshoot of my Vulcan blood, then attack becomes even more imperative.
McCoy: War is never imperative.
Spock: It is for them, Doctor. Vulcan, like Earth, had its aggressive colonizing period, savage even by Earth standards. If Romulans retain this martial philosophy, then weakness is something we dare not show.

McCoy: Something I seldom say to a customer, Jim. In this galaxy, there's a mathematical probability of three million Earth-type planets. And in all of the universe, three million million galaxies like this. And in all of that, and perhaps more, only one of each of us. Don't destroy the one named Kirk.

Romulan Commander: I regret that we meet in this way. You and I are of a kind. In a different reality, I could have called you friend.
Kirk: What purpose will it serve to die?
Romulan Commander: We are creatures of duty, Captain. I have lived my life by it. Just one more duty to perform.

The Enterprise is in contact with Outpost 4 when the Romulans destroy it. Commander Hansen switches the video so Kirk can see the attacker. After the ship fires, the output is clearly destroyed; we can see Hansen's office go white and fade away. Then the view switches back to the attacking Romulan ship for a few seconds, as if the outpost was still somehow able to send. (This was doubtless done for dramatic effect, so the viewer could see the ship fade to invisibility - but it still counts as a plot hole.)

Changes in the remastered version
In addition to the expected clean ups of video and audio, there were several significant changes: the "phasers set for proximity blast" appeared more like photon torpedoes when fired, and like large white explosions on detonation. Here they were replaced by a bluish and more realistic looking detonation. The Romulan torpedo was replaced by a more dynamic effect, and the Romulan bird of prey was replaced by a more realistic looking CGI effect, as were its fades into and out of visibility.