“The Enterprise Incident”, first aired on 27 September 1968, shifted Star Trek into Cold War-style espionage, sending Kirk, Spock and McCoy on a covert mission to steal the Romulans’ cloaking device. It matters because the episode showed the Federation compromising its principles under official orders, establishing a morally ambiguous template for later stories about intelligence operations and Section 31.

Starfleet deliberately violated the Neutral Zone and fabricated Kirk’s apparent death and betrayal, while McCoy altered Kirk’s appearance so he could infiltrate a Romulan vessel. The episode also humanised the Romulans as a rival power rather than simple villains, with Joanne Linville’s unnamed commander portrayed as intelligent, formidable and romantically intriguing to Spock.