Rubicun III is the idyllic world of the Edo, where any crime in a hidden punishment zone carried the death penalty.
Rubicun III is a pleasant, newly discovered planet inhabited by the Edo, a friendly and uninhibited people. In Justice the Enterprise stopped there for shore leave, and the crew soon learned that Edo law punished any offence committed in a randomly chosen punishment zone with death.
Wesley Crusher accidentally broke a greenhouse inside such a zone and was sentenced to die. The planet was also watched over by a powerful being in orbit that the Edo regarded as their god. Picard had to balance the Prime Directive against saving Wesley before the entity finally let the crew leave with him.