Geordi's close friend — sort of like his big sister — Susanna Leijten (Maryann Plunkett) comes aboard to report that the other officers of an away mission they all had been on several years earlier have recently gone missing. The Enterprise tracks a shuttle from one of the missing crewmen to the original mission's planet. They find no trace of the missing crewman, but Susanna, and then Geordi, begin having strange medical symptoms that draw them to the planet. It turns out they were both afflicted, on that years-ago mission, by an alien influence that is now rewriting their DNA. When Susanna starts transforming into an alien, Crusher must race to find a way to stop it before Susanna and Geordi are both lost.
"Identity Crisis" is a tolerable hodgepodge of stories. It's a merging of various familiar devices including (1) Starfleet officers who have gone missing, (2) an old close friend we've never heard of before, (3) a medical mystery, (4) an alien parasite, (5) a holodeck investigation, and (6) Fun With DNA [TM]. Actually, this might be the best Fun With DNA episode on record, since Fun With DNA generally makes me want to retch. Brannon Braga apparently decided that if "Identity Crisis" was the starting point then "Threshold" was the logical extension, but I digress.
The most interesting scenes involve Geordi on the holodeck trying to put together clues from a video recording of the original mission. Hey, look! There's a shadow from someone who isn't there! What is that? Before long, Geordi has become an invisible alien himself, and goes careening through the ship with the aid of his own personal biological cloaking device, like the alien in Predator.
The first half of the story does a pretty good job of creating a sense of mystery about what's going on, as Susanna slowly, psychologically melts down. And the second half of the show — which focuses on the close friendship between Geordi and Susanna as they try to save each other — works emotionally, even as the science goes off the deep end (transforming people into aliens and back without killing them and in such a way that even their hair looks the same, etc.). I can't recommend "Identity Crisis," but there are things about it that work in spite of itself.