A light-craving virus spreads through the Enterprise after a visit to an abandoned Illyrian colony, and Number One reveals her own Illyrian heritage to save the crew.
The Enterprise investigates an abandoned colony of Illyrians, a genetically modified people barred from the Federation. Away team members bring back a virus that makes its victims crave light, and it spreads through the ship. Number One is immune because she is secretly Illyrian, and she reveals it so that a cure can be made from her blood.
Trapped on the planet by an ion storm, Pike and Spock learn the colonists had been trying to undo their modifications. Afterward Number One offers to resign, and Pike refuses. The virus slipped past the transporter's filters because Dr. M'Benga uses an old transporter buffer to keep his terminally ill daughter Rukiya in stasis while he searches for a cure.