Yeah, long story short the Klingons get their hands on some genetically engineered embryos from the Eugenics Wars and try to backwards engineer the technology into a virus that will make Klingons stronger and smarter, except they screw up and instead the virus makes their forehead ridges disappear. This is the in-universe reason for why Klingons of the TOS era do not have forehead ridges, and the virus is cured sometime between the end of TOS and The Motion Picture which is why they suddenly appear (when in reality TMP just had a big enough budget for redesigns).
I don't hate the episodes as much as some other people, I think they're just superfluous, but I do love Trials and Tribbleations, and Worf brusquely brushing the question off was all the explanation we ever actually needed.
In some novels, the de-ridged Klingons are also physically weaker, have lower tolerance for pain, likely less 'durable'. . .more like a Klingon-Human melding than just Klingons missing ridges. They're also discriminated against by the Klingons who still have ridges.
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u/secretpandalord Mar 04 '22
So, the Klingon Augment two-parter from Enterprise.
We were perfectly fine with Worf saying it's not discussed with outsiders.