S3, ep 9 “Trusted Sources”

The crew of the Cerritos is performing their duties during “Operation Check-Up” and upon checking in on the planet of Brekka, who had a drug-dealer like relationship with the Ornarans (TNG s1e22, Symbiosis), discover the population is almost non-existent, and the only lead are the Breen, who ambush the crew and Brekkian civilians.