Continuing the Klingon Duology from "Affliction," this is indeed the story that finally explains the smooth- versus ridge-headed mystery that popped up between the original series and every other incarnation of Star Trek.
Phlox gets more interesting stuff to do as he is forced desperately to solve a problem that would have made the consequences of the Klingons becoming obsessed with the Augments from earlier in the season, but again his thunder is stolen by Reed's reluctant involvement with Section 31 (as the shadowy Jack Bauers of Deep Space Nine fame) and a thrilling Speed-like action sequence wherein Trip comes back to the Enterprise as it connects to the Columbia at Warp 5 so they can undo Klingon sabotage.
That sequence absolutely steals the episode. It's one of the most exciting moments in any Star Trek episode or movie (perhaps yet another indication of just how desperate the series had gotten to win viewers back, which...didn't happen).
I would almost, almost give the episode the full-recommendation treatment based on that alone. The historic value of the Klingon Duology also makes it worth viewing...but again, I really wish Phlox had been given better material, and he's technically at the center of events. The fact that Reed and Trip steal the episode from Klingons crewing up their genetics and metaphorically going bald is what makes it easy to say, "Close, but no cigar."
Still, if you're game, definitely watch.
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Memory Alpha summary.