“Extinction” is episode three of season three of Star Trek: Enterprise.
An alien is running through a forest from hunters in environment suits. When they corner him, they ignite flamethrowers.
T’Pol is in her quarters when Trip arrives for some neuropressure with some Georgia peaches he had in stasis. He wants her to try one. She does. T’Pol says this doesn’t make up for the two sessions he missed. Trip says he’s been busy. It’s not like he didn’t want to come back; after their last session he slept like a baby for the first time in months. T’Pol has Trip lie down and starts. He asks if it’s safe and if anyone has tried this on a human before. Not to her knowledge. The risk of paralysis is minimal.
Then the captain contacts T’Pol; he wants her to join him in the command centre. He shows her another species of Xindi; Phlox thinks it’s descended from an arboreal primate. The captain has been working with the Xindi database they obtained in the previous episode, “Anomaly”, and has reconstructed the course of the ship it came from. He’s found the last planet the Xindi visited before being attacked by the marauders and it’s only one day away. T’Pol says they don’t know if the primates were involved in the attack. The captain says they don’t know they weren’t.
Enterprise arrives at the planet. T’Pol detects lots of plants but no humanoid lifesigns. Reed has detected a metal signature that looks like a landing craft. No biosigns near it. Captain Archer, Reed, T’Pol and Hoshi head down in a shuttlepod. They find the landing craft and T’Pol says it’s been there about two weeks. Right about the time the Xindi arrived. The captain says it is Xindi; he recognises it from their database. Hoshi has found a body; not Xindi, or at least not one of the three they know of, according to T’Pol. Reed finds another.
They start searching the area. T’Pol notices her arm changing, then lumps appear on her face. She speaks to Reed, who is changing. As is the captain. T’Pol contacts Enterprise for a security team and says they’re being altered. The other three now all resemble the alien from the beginning, and also speak another language. T’Pol runs away but falls to the ground.
The other three are carrying T’Pol on a makeshift litter when she comes around. T’Pol manages to convince the captain to untie her, then gets Hoshi to relinquish her universal translator. T’Pol wants them to talk. Naturally, they don’t.
On Enterprise, Phlox is confirming that one biosign is Vulcan but the other three are definitely not human. The only reason Trip hasn’t sent down a team is because Phlox said he needed more time to analyse the data. Phlox suggests that perhaps what T’Pol said about the team changing was literal; something on the planet has transformed them. The three nonhumans might be their team. In the Expanse, they have to consider that natural laws may be as changeable as physical. Trip is going to go down with environment suits and biohazard protocols, and use the decon chamber when they get back.
T’Pol has managed to get the others to speak so the translator now knows their language. The want to find Urquat; their people are there. Maybe they can explain why the three of them don’t remember what happened. T’Pol wants them to contact Enterprise. Hoshi says she’s not one of them and can’t be trusted. The captain doesn’t know what T’Pol is.
The three have been transformed into another lifeform and the aliens from earlier have quarantined the planet because of the virus that does this. And their containment protocols are quite stringent. When they arrive, they want to kill everyone infected. So, a cure needs to be created and the other aliens stopped from killing the away team. Transforming into another species is not the most original thing (this happened in the TNG episode, “Identity Crisis”, in which Geordi was the one transformed; this episode was directed by LeVar Burton, who played Geordi.