Htydjtk

I never quite understood the disconnect between the Klingon appearance in the original series and the others (TNG and beyond) until I read recently on Wikipedia:

A canonical explanation for the
change[Klingon appearance] was given
in a two-part storyline on Star Trek:
Enterprise. The two episodes,
"Affliction" and
"Divergence", aired in February
2005. An earlier story arc featured the Augments, genetically-engineered
humans left over from the Eugenics
Wars of the late 20th century, and who
were defeated by Captain Jonathan
Archer and the Enterprise in Klingon
space. The Klingon High Council fears
that Starfleet was developing armies
of Augments; after gaining access to
genetic material from the Augments,
the Klingons perform experiments to
increase their own intellect and
strength. The experiments turn
disastrous when a flu strain mutates
and becomes a deadly plague that
spreads across the Empire, causing
physical changes resulting in the
afflicted bearing a TOS-era
appearance. Dr. Phlox of the
Enterprise formulates a cure for the
virus, but the physical alterations
remain in the populace and are
inherited by offspring. Phlox
indicated that "someday" the physical
alterations could be reversed.

But in the TNG episode "Rightful Heir," the cloned Kahless has the same appearance as the "mutated" Klingons. Kahless lived long before the experiment in the canonical explanation. Is this a plot discontinuity?

Kahless in "Rightful Heir" had the original, non-mutated appearance (you can see a screen capture on Memory Alpha for confirmation).

Prior to "Affliction" (including the Star Trek: Enterprise pilot, "Broken Bow"), Klingons had the same appearance as they did in TNG and beyond. The idea is that they had appearance A before "Affliction", appearance B until TNG, and went back to appearance A sometime before TNG.

So, Kahless, having existed before the events of "Affliction", was unaffected. However, there is a plot descrepency in the original series episode "The Savage Curtain" where there is apparently an appearance B version of Kahless (again, click into Memory Alpha for confirmation).

But this is retconned to being an approximation of what the appearance B Klingons thought he looked like, not the actual Kahless.

One explanation I read (I believe in the Star Trek Encyclopedia) is that the creature from The Savage Curtain created the opponents from Kirk and Spock's imaginations (just as he did their allies), and since Kirk and Spock knew only Klingons of the TOS variety, then that is how they imagined Kahless. Sorry for the lack of sources on this one.

The original Kahless, having lived eons before any series takes place, was therefore of the ridged forehead variety, and his clone remained thus. The thing that's nice with this explanation is that whether or not you consider the Enterprise retconning, it works.

This is kind of a retcon, or retroactive continuity, where later episodes (or, in this case, a later series in the same canon) alters perviously established facts. In this case, the Enterprise episode retcons the TNG episode in a way that says, what you think you saw -- you didn't.

So, even though the Kahless clone looks like the mutated Klingons, since Enterprise retconed the Klingon story line with new information, he really didn't. Wikipedia has a piece on retcons.

I always just read it as Ridges being a prior evolution of Klingon, Unridged being the more evolved form and the virus causing their physical appearance to revert to a less evolved state.