Hey, is that a targ?
Or "Welcome to the Klingons!" In this, the 26th episode of the first season of Star Trek, the most iconic aliens in the history of the franchise make their debut.
However, their language doesn't, nor do the forehead ridges... more on that later.
When war breaks out between the United Federation of Planets and the Klingon Empire, the Enterprise is sent to the peaceful planet of Organia to prevent the latter from getting their hands on a key strategic location. However, the local leadership is completely unwilling to allow Kirk to help them and in due course, the Klingons occupy the planet.
I'll start by pointing out that McCoy isn't in this episode, nor is Scotty. It would have been interesting to see how Bones reacted to the bat'leth brigade here, although those weapons don't turn up until The Next Generation.
It is important to place this story in its historical context. "Errand of Mercy" aired on 23 March 1967. The Cuban crisis was 4 1/2 years previously and there would be a serious set of clashes between the USSR and China in 1969. The Six-Day War would occur that summer. It was a rather tense time in global history, to put it mildly.
So with the Federation cast as the Americans, it becomes a fairly simple case of seeing the Klingons as the Soviet Union or Red China in this series... but a bit less so in their debut.
I've previously mentioned that the Second World War was still very much in living memory for those making the show and the Klingons here are portrayed as rather akin to the Nazis, taking hostages and making the local government implement their orders. Commander Kor, the military governor here, would not be out of place in a SS uniform (the Klingons wear jackboots), calmly drinking champagne as he shoots a captured resistance fighter with a Luger P08 pistol. If this was broadcast today, you'd almost certainly have a helpline number given out at the end of the credits; there would be those watching who had encountered real life 'Klingons'.
Kirk and Spock therefore end up acting as a mini-resistance against the Klingons, blowing up a munitions dump with a 'sonic grenade', which is probably no relation to a sonic screwdriver. It's a rather enjoyable tale of 'derring-do' reminiscent of a comic book at times.
Knowing the twist in this episode actually makes it better, because you can see all the little details. That said, the twist does raise further questions related to future events in the franchise and also is notable in a prediction becoming entirely true.
Anyway, back to the Klingons. They would become the main antagonists for the Federation in TOS, mainly because it was easier to do the make-up than the Romulans. John Colicos (half Greek, half Canadian - who also played Count Baltar in the original Battlestar Galactica and Mikkos Cassadine in General Hospital in a long career) who played Kor, came up with the distinctive look, intending to invoke Genghis Khan, although Khan lacked the crazy eyebrows. Many of the Klingons just look like they've used the wrong sort of bronzer to me... there are of course unfortunate racial implications, especially with the Fu Manchu style moustache of Kor, although he doesn't use a silly accent.
However, the Klingons would rise far above that (indeed many Klingons don't have dark make up in TOS, although all the key ones were played by white actors), even if it wouldn't be for a while. The forehead ridges don't come until the movies; I would also point out the Klingon ships seen here are CGI add-ins for the remastered version.
A classic debut for a classic race.