The officers who keep the law among the Edo of Rubicun III by visiting randomly chosen punishment zones, where any offence at all carries an immediate sentence of death.
Mediators are the only police the Edo need. Each day a different area is secretly designated a punishment zone, and any breach of the law discovered there by the mediators is punished by a lethal injection given on the spot. Since nobody knows where the zone will be, nobody breaks the law.
In Justice, the Enterprise crew are enjoying shore leave among the affectionate Edo when Wesley Crusher falls into a bed of new plants while catching a ball. Two mediators arrive and prepare to carry out the sentence, and are held off only by Worf and Yar.
The Edo leaders Rivan and Liator sympathise but insist the law allows no exception. Picard must also answer to the orbiting entity the Edo worship as God, and persuades it that justice cannot rest on absolutes.