Star Trek has spent decades exploring strange new worlds, seeking out new life and new civilizations, and occasionally doing a little espionage along the way. Long before Section 31 started doing Starfleet’s dirty work, Captain Kirk (William Shatner) and Mr. Spock (Leonard Nimoy) discovered that saving the Federation sometimes required borrowing a few tricks from Ethan Hunt.

“The Enterprise Incident,” which first aired on Sept. 27, 1968, dropped the crew of the Enterprise into the middle of a Cold War-style spy story involving Romulans, secret identities and one extremely valuable piece of technology. Nearly six decades later, it plays like a blueprint for the espionage stories Star Trek would eventually embrace.

But what makes “The Enterprise Incident” endure isn’t simply the novelty of watching Kirk and Spock sneak around for an hour. It ventures into surprisingly murky territory for Star Trek, complicating who the good guys and bad guys are while introducing an unnamed Romulan commander who nearly steals the entire episode.

Star Trek turned its heroes into spies

Kirk and Spock aren’t rogue agents operating behind Starfleet’s back. Quite the opposite: Starfleet sent them there. The Federation deliberately violates the Neutral Zone and sends its flagship on a covert mission to steal another power’s military technology, complete with a cover story allowing Starfleet to deny responsibility if everything goes wrong.

That cover story involves convincing the Romulans that Kirk has become unstable and crossed into their territory on his own. Spock seemingly betrays him, and when Kirk attacks his first officer, Spock appears to kill him with a “Vulcan death grip.” McCoy even confirms the death. There’s just one problem: The Vulcan death grip doesn’t exist. Spock apparently invents a new piece of Vulcan lore on the spot, McCoy plays along, and Kirk gets to continue being considerably less dead than the Romulans believe.

McCoy then surgically alters Kirk to look Romulan, pointy ears and all. That allows Kirk to sneak aboard an enemy ship and steal the cloaking device himself. It’s all wonderfully ridiculous, but there’s something surprisingly stealthy underneath it. Kirk, Spock and McCoy aren’t bending Starfleet’s rules. They’re following orders. “The Enterprise Incident” understood decades before Section 31 that even the Federation occasionally wants something badly enough to get its hands dirty.

The Romulans aren’t just the bad guys anymore

Paramount

The spy story only works because “The Enterprise Incident” doesn’t treat the Romulans like generic villains. “Balance of Terror” had already established them as formidable adversaries, but here we spend time aboard their ships and watch Kirk and Spock interact with them as people rather than targets on a viewscreen.

From the Romulans’ perspective, they’re not exactly the bad guys either. Kirk is the one who crossed their border. A Federation starship has illegally entered Romulan territory and its captain appears to be behaving like a lunatic. Wanting to know what the hell is going on seems pretty reasonable.

That ambiguity makes the conflict less about defeating an enemy than outmaneuvering a rival. Both sides understand intelligence, military technology and leverage, and neither has a monopoly on questionable behavior. It also helps that the person commanding the Romulan forces isn’t some one-dimensional villain. She’s intelligent, confident and more than capable of matching Spock intellectually, which he certainly notices.

The most interesting character doesn’t even have a name

Paramount

For all the fun of watching Kirk play secret agent, “The Enterprise Incident” belongs to Joanne Linville’s Romulan commander. She is never given a name, but she gets something arguably more valuable in a Star Trek episode: Mr. Spock’s undivided attention.

Spock doesn’t exactly have an extensive romantic résumé, which makes watching him essentially seduce a Romulan commander in the name of interstellar espionage one of the episode’s stranger pleasures. The commander is clearly attracted to him, and Spock recognizes that her interest can help the mission. Their conversations gradually move away from politics and toward something much more personal, keeping her distracted while Kirk goes searching for the cloaking device.

Yet Leonard Nimoy makes the whole thing more interesting than a simple seduction. Spock appears to genuinely respect her, and the episode leaves enough ambiguity around his feelings to make you wonder where the performance ends. For someone supposedly running a con, he seems awfully committed to the role.

Eventually, the commander realizes what’s happened. Kirk gets the cloaking device, Spock escapes and she winds up aboard the Enterprise. After discovering that her potential new Vulcan boyfriend was actually helping steal one of her empire’s most valuable military secrets, she does what apparently no one else in Star Trek ever thought to do: She slaps Spock.

Linville ensured the commander was memorable regardless of whether the episode bothered to give her a name. She would later become an acting teacher at the Stella Adler Academy, where her students included future Oscar nominee Mark Ruffalo. Not bad for someone who went toe-to-toe with Spock and nearly came out on top.

“The Enterprise Incident” still has all the ingredients of a great Star Trek spy story: questionable orders, secret identities, double-crosses, sympathetic enemies and Mr. Spock flirting with someone he probably shouldn’t.

Sometimes exploring the final frontier requires going undercover.