An incurable disease attacking the brain and nervous system, which is killing the cyberneticist Ira Graves when the Enterprise answers a distress call from his home on Gravesworld.
Darnay's disease appears in the Next Generation episode The Schizoid Man. The Enterprise sends an away team to the isolated world where Dr Ira Graves lives with his young assistant Kareen Brianon, who has called for help without his knowledge.
The Vulcan doctor Selar confirms that Graves is in the final stage of the disease, which also accounts for his irritability and pain. No treatment exists and he has very little time.
Graves, who regards himself as the teacher of Data's creator Noonien Soong, has prepared for this. Before he dies he secretly transfers his mind into Data, and the android's increasingly arrogant and jealous behaviour occupies the rest of the story.