"Identity Crisis" was the 18th episode of the 4th season and was written by Brannon Braga.
The story begins with the investigation by Geordi LeForge and Lt. Commander Susanna Leijten of former fellow crewmates from the USS Victory who have gone missing after taking shuttlecrafts back to a remote planet they once explored.
As they investigate, Leijten begins exhibiting signs of anxiety and confusion as they find unusual footprints near one of the shuttlecrafts on the planet. She doesn't want to leave and is brought up by transport to sick bay. There, she shows signs of metamorphosis. Her skin absorbs light and her eyes become sensitive to it.
Knowing all the former crew from that mission have gone missing back on the planet, Geordi is aware he may be next but is determined to keep working to find out what is causing it. He recreates a visual log in the holodeck to try to explain a shadow that doesn't match any of the former crew. But by then, Geordi too is beginning the metamorphosis and then becomes undetectable by the ship's standard tracking tracers.
Meanwhile, the doctor finds, what she assumes, is a parasitic creature attached to Leijten's thymus gland and it's spreading genetic instructions to change the body. After it's removal, Susanna begins to return to normal and attempts, along with the away team, to locate Geordi.
Geordi manages to transport himself to the planet and the crew detects him with ultraviolet light. His friend Susanna coaxes him back gently because the creature makes the host fearful and act purely on instinct.
Dr. Crusher performs the surgery, saving Geordi from a permanent transformation.
One thing I remember about this episode when it first aired was watching it with my mother, who is not a fan of the sci-fi genre. Her response to it: "That was pretty good." Pretty impressive critique for her!