'Homefront'
Season four, episode eleven
This claustrophobic episode, and it's sequel 'Paradise Lost', are one of the first inklings in the DS9 series that the mood is going to turn much darker as the show develops, taking it away from the previously conceived idea that life in the twenty-fourth century Trek verse is a perfect utopia. The episode sees Sisko and Odo arriving on Earth where Starfleet Command has asked for their expertise in helping Starfleet Security prepare contingencies to protect Earth as they believe the Founders have have infiltrated the Federation.
Aside from seeing more of Sisko's personal life in the form of meeting his chef father Joseph, what made this episode so good was watching as Federation Earth seemed to crumble as they prepared to face this new threat from within. After nearly four centuries on peace on the planet, I imagine the high levels of paranoia, fear and anxiety as people began to distrust even those close to them is exactly how humans would react despite claims they have moved passed such immature behaviour expected more from a 'barbaric' twenty-first century human. It also has hints, which link to how the Federation would eventually win the war, over why unity is more important than division. After all, the Founders were able to ravage the Alpha Quadrant by dividing up old alliances and casting doubt among various races.