What is a Klingon supposed to look like? Depending on your favorite era of Star Trek, your answer might be totally different. To the casual fan, there are only two kinds of Klingons — regular Klingons, and Discovery Klingons. Hardcore devotees know there's a remarkably wide spectrum of alien make-up and the history of these bumpy foreheads is well... bumpy.
After making a huge break with TNG-era Klingons in Season 1 of Star Trek: Discovery, the show is poised to reconcile three major types of Klingons seen in the Prime Universe canon. Here's why Discovery Season 4 might be poised to bring back the Klingons TNG fans remember, and why that could actually matter for the story.
A brief history of Klingon foreheads — When the Deep Space Nine gang Forrest Gumps themselves into The Original Series in the episode "Trials and Tribble-ations," Odo asks Worf (a Klingon with a bumpy forehead) why the classic TOS Klingons have smooth foreheads. Worf says "we do not discuss it with outsiders." In 1996, this was the extent of in-canon explanation for the sudden appearance of Klingons with ridged foreheads in 1979's Star Trek: The Motion Picture. Trek creator Gene Roddenberry went on record more than once saying that he wanted fans to just accept the fact that TOS didn't have the budget to show Klingons the right way, and you should basically imagine Klingons always having looked like that. TNG and DS9 ran with this concept, and even when Klingons from TOS reappeared on DS9 — Kor (John Colicos), Kang (Michael Ansara), and Koloth (William Campbell) — all had been restyled to reflect the then-contemporary Klingon "look." No on-screen explanation was ever offered as to why these exact same Klingons looked different, 70 years later. To put this in perspective, this would be like if Worf (Michael Dorn) had appeared in Star Trek: Picard Season 1, but had been inexplicably been re-made to have the full-head Klingon makeup from Star Trek: Discovery. In other words, it's the kind of visual retcon that people went along with in the '90s, but would have not been okay with in 2020.
In canon, the official explanation for why there were smooth-headed Klingons finally came in an Enterprise episode oh-so-subtly titled "Divergence." This episode revealed that in the 22nd Century (a hundred years before TOS and two hundred before TNG) a segment of the Klingon population was infected with a virus from genetically-engineered humans. At present, this remains the only episode that features smooth-headed Klingons and ridged-forehead Klingons, at the same time, and not using any time travel.
Disco Klingons muddied the waters — When Discovery Season 1 debuted, the Klingons again had a new look. But, unlike the shoulder-shrug from fans in the '70s and '80s, people freaked out. These Klingons had full-on monster heads, clawed hands, and were almost universally, bald. Discovery later suggested that the Klingons were only bald because they had shaved their heads for war and in Season 2 L'Rell (Mary Chieffo), and other Klingons grew their hair out. Superficially, giving the Klingons hair in Discovery Season 2 moved them closer to the TNG and movie-era Klingons, but they still look really different, and we've never really been told why. One theory is that the vast majority of Klingons we saw in Discovery came from T'Kuvma's sect of super-old Klingons, who had been out of touch with other Klingons for a long-ass time and literally living on a ship called the Sarcophagus. (Think of this kinda like the Star Trek version of The Mandalorian's "Children of the Watch.")
Although Discovery has never outright acknowledged the existence of the smooth-headed TOS Klingons or the TNG "normal" Klingons, the series has a tipped-its hat to both in roundabout ways. In Season 1, Ash Tyler was revealed to be a Klingon sleeper agent, surgically altered to look like a Human. This exact same plotline was borrowed from the goofy TOS episode "The Trouble With Tribbles," which, was, of course, the episode the DS9 crew traveled to in "Trials and Tribble-ations," allowing Worf to have his cryptic line about the human-ish Klingons existing within the regular Klingon culture.
Discovery is now a sequel to TNG and DS9 – Discovery Season 3 was the first DISCO season to not feature a single Klingon. In fact, the Klingons were only mentioned in the context of the Mirror Universe two-parter "Terra Firma," which, also took place in the past. But the bulk of Discovery Season 3, and going into Season 4, takes place in the 32nd Century. Season 3, spent a good amount of time revisiting familiar Star Trek planets, including Trill and the planet formerly known as Vulcan – Ni'Var. In both cases, Discovery was paying respect to canon from TNG and DS9. In fact, the Discovery episode "Unification III" is a direct sequel to the TNG episodes "Unification I" and "Unification II." Visually, Discovery acknowledged this canon by showing archival footage of Spock (Leonard Nimoy) during his time with Picard on the Romulan homeworld.
Interesting detail there: Picard and Data infiltrated Romulus with the help of the Klingons. For Discovery characters, those types of Klingons wouldn't look much like the monster-head Klingons they're used to. So, the question is, what do the Klingons of the 32nd century look like? If Discovery wants to keep consistent with being a sequel to the TNG/DS9/Voyager era of Trek, then it seems like the appearance of the Klingons in Season 4 could mean a full-on renaissance of the Worf-style Klingon foreheads most fans seem to prefer.
There are broader consequences at stake, too. Now that Michael Burnham is the Captain of one of the Federation's most powerful ships, how do the Klingons feel about that? Does their history tell them that she started the Klingon war in 2256 by murdering T'Kuvma? Or, do they remember her as the person who bartered peace with L'Rell in 2257? Either way, Burnham is in a position much like Kirk was in The Undiscovered Country. She's got some beef with Klingons, and they might have some beef with her, too.
Which could lead Discovery to its most interesting revelation. If the Cardassians were Federation members by the time of the Burn...then did the Klingons join, too?
Star Trek: Discovery Season 4 is currently filming.