In "Extinction" an away mission to a jungle planet goes terribly wrong when the crew are infected by a mutagenic virus that transforms them into primitive ape-like creatures. While T'Pol attempts to convince them to return to Enterprise, they are hunted by another group of aliens desperate to eliminate the spread of the virus by any means necessary.
As I mentioned earlier, the base concept of the episode is actually a good one: a species that's dying out creates a virus that transforms aliens into that species. It's a concept ripe for drama and interesting storylines - it's just a shame they picked the storyline that they did. There are a few bits and pieces worth watching: Trip and T'Pol get another scene to further their budding romance, and it's one that works much more smoothly than the ones in "The Xindi". It's just a shame that the scene had to be wedged into an episode that otherwise pretty unwatchable.
We're three episodes into Season 3: precisely one of those episodes has been good. That leaves it with a quality ratio of 33% - not promising.