“Identity Crisis” is episode eighteen of season four of Star Trek: The Next Generation.
The episode opens with a recording of an away team from the USS Victory. Geordi is one of the members of the team, who are investigating why everyone at an outpost on Tarchannen III just vanished. One has found a torn and empty uniform, but no footprints leading away.
This is a recording which is paused by the person who headed the away team, now a lieutenant commander, Susanna Leijten. She’s showing it to Captain Picard, Commander Riker and Geordi. Of the other members of the away team, Brevelle deserted from Starbase 112 last week after being ill for several days, Mendez was seen on the Ares an hour before she disappeared and Hickman stole a shuttle that was spotted heading to Tarchannen III. The original mystery was never solved; 49 people just disappeared. Of the away team sent to investigate, only Leijten and Geordi are left.
The Enterprise sets a course to intercept the shuttle and Geordi and Leijten are catching up, and admitting their worries, in Ten Forward when Riker summons them to the bridge as the shuttle has been located. The shuttle isn’t responding to hails and it’s heading for the atmosphere of Tarchannen III at a dangerous approach. It’s too far away for transporters or tractor beams and it explodes in the atmosphere. Worf detects two more Federation shuttles on the ground. Leijten wants to accompany the away team.
The away team beams down next to the shuttlepod taken from the Ares. Leijten finds some barefoot, and not human, footprints and follows them. There are no lifesigns but Worf is certain they are being watched. Geordi has found a torn uniform in the shuttle. Leijten doesn’t respond to his comm request and he goes looking for her, following the same prints. Leijten calls him; the others are here, she can feel it. Geordi can’t detect anything. Leijten starts to walk away and Geordi has to grab her and request an emergency beam out to sickbay.
Leijten wakes up in sickbay. Dr Crusher tells her she had a histamine response, but the doctor doesn’t know what caused it. Leijten isn’t to leave the Enterprise until the doctor has some answers, and the captain agrees with her. Leijten leaves with Geordi to take a look at Data’s preliminary report. Dr Crusher tells the captain that Leijten’s bioscan says she is in perfect shape. Which, Picard comments, is exactly the same as weas said about two of the missing people.
Leijten is telling Geordi they need to send dozens of away teams down. However, her hands are shaking badly and involuntarily. Data has found alien skin cells on the torn uniform and the footprints come from no known Tarchannen lifeform. No tracks were found in the original investigation, but Data suggests they could have been blown away in the frequent sandstorms.
Data and Dr Crusher are searching for something to match the prints. On the doctor’s question, Data says he is an android; he doesn’t get anxious. But admits he is strongly motivated to solve this. Geordi and Leijten are in engineering going over the original logs when Leijten says this is a waste of time; they should get back to the planet. She is too desperate to leave. Tarchannen III is down there, waiting for them. Then she has a fit and collapses, with purple veins on her skin and partially fused fingers.
In sickbay, Leijten has transformed much further. Geordi is reassuring her but she says it’s inside her and winning. The doctor tells Picard that Leijten’s altered skin cells are almost identical to the ones found on the torn uniform. The missing people have been transformed into another species. Leijten is already transforming and it’s likely only a matter of time before Geordi starts to as well. And if they transform too far away from human, will it be possible to change them back?