Nelvana III is a planet in the Neutral Zone where a Romulan defector claimed a secret base was being built, a claim that proved to be a trap.
Nelvana III lies inside the Romulan Neutral Zone. In The Defector a Romulan who first called himself Setal and was then revealed as Admiral Alidar Jarok warned Picard that the Romulans were building a secret base there that could support a fleet ready to strike at the Federation.
Picard took the Enterprise into the Neutral Zone to check, and found nothing on Nelvana III. The story had been fed to Jarok by his own government to lure the Enterprise into a trap set by Tomalak. Picard escaped thanks to Klingon ships he had arranged to have nearby, and Jarok, his defection wasted, took his own life.